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Niels Bohr 

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Niels Bohr 

London • Chapman & Hall, Limited 










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- -X 2 8 SEP 1987 


Copyright © 1958 by Niels Bohr 

All rights reserved. This book or any part 
thereof must not be reproduced in any form 
without the written permission of the publisher. 

Library of Congress Catalog Card Number: 58-9002 
Printed in the United States of America 


r^his collection of articles, written on various occasions within 
_L the last 25 years, forms a sequel to earlier essays edited by 
the Cambridge University Press, 1934, in a volume titled Atomic 
Theory and the Description of Nature. The theme of the papers is 
the epistemological lesson which the modern development of atomic 
physics has given us and its relevance for analysis and synthesis in 
many fields of human knowledge. The articles in the previous edition 
were written at a time when the establishment of the mathematical 
methods of quantum mechanics had created a firm foundation for 
the consistent treatment of atomic phenomena, and the conditions 
for an unambiguous account of experience within this framework 
were characterized by the notion of complementarity. In the papers 
collected here, this approach is further developed in logical formula- 
tion and given broader application. Of course, much repetition has 
been unavoidable, but it is hoped that this may serve to illustrate 
the gradual clarification of the argumentation, especially as regards 
more concise terminology. 

In the development of the views concerned, discussions with former 
and present collaborators at the Institute for Theoretical Physics in 
the University of Copenhagen have been most valuable to me. For 



assistance in the elaboration of the articles in this volume, I am espe- 
cially indebted to Oskar Klein and Leon Rosenfeld, now in the uni- 
versities of Stockholm and Manchester, as well as to Stefan Rozental 
and Aage Petersen at the Copenhagen Institute. Also I should like 
to extend my thanks to Mrs. S. Hellmann for her most effective help 
in the preparation of the articles and the present edition. 

Niels Bohr 
August 1951 




Address at the opening meeting of the International Congress 
on Light Therapy in Copenhagen, August 1932. Printed in 
Nature, 131, 421 (1933). 



Address at the Physical and Biological Congress in memory of 
Luigi Galvani, Bologna, October 1937. 


Address at the International Congress of Anthropological and 
Ethnological Sciences in Copenhagen, delivered at a meeting 
in Kronborg Castle, Elsinore, August 1938. Printed in Nature, 
143, 268 (1939). 




Contribution to Albert Einstein: Philosopher-Scientist. The 
Library of Living Philosophers, Inc., Evanston, Illinois, vol. 7, 
1949, p. 199. 


Address delivered at a conference in October 1954 in connection 
with the Bicentennial of Columbia University, New York. 
Printed in The Unity of Knowledge, Doubleday and Co., New 
York, 1955, p. 47. 


Address delivered at a meeting of the Royal Danish Academy 
of Sciences in Copenhagen, October 1955. 


Article completed in 1957 and based on a Steno Lecture in the 
Danish Medical Society, Copenhagen, February 1949. 


j'he importance of physical science for the development of gen- 
ii, eral philosophical thinking rests not only on its contributions to 
our steadily increasing knowledge of that nature of which we ourselves 
are part, but also on the opportunities which time and again it has 
offered for examination and refinement of our conceptual tools. In 
our century, the study of the atomic constitution of matter has re- 
vealed an unsuspected limitation of the scope of classical physical ideas 
and has thrown new light on the demands on scientific explanation 
incorporated in traditional philosophy. The revision of the founda- 
tion for the unambiguous application of our elementary concepts, 
necessary for comprehension of atomic phenomena, therefore has a 
bearing far beyond the special domain of physical science. 

The main point of the lesson given us by the development of 
atomic physics is, as is well known, the recognition of a feature of 
wholeness in atomic processes, disclosed by the discovery of the 
quantum of action. The following articles present the essential 
aspects of the situation in quantum physics and, at the same time, 
stress the points of similarity it exhibits to our position in other fields 
of knowledge beyond the scope of the mechanical conception of 


nature. We are not dealing here with more or less vague analogies, 
but with an investigation of the conditions for the proper use of our 
conceptual means of expression. Such considerations not only aim 
at making us familiar with the novel situation in physical science, but 
might on account of the comparatively simple character of atomic 
problems be helpful in clarifying the conditions for objective descrip- 
tion in wider fields. 

Although the seven essays here collected are thus closely intercon- 
nected, they fall into three separate groups originating from the 
years 1932-1938, 1949, and 1954-1957, respectively. The first three 
papers, directly related to the articles in the previous edition, discuss 
biological and anthropological problems referring to the features of 
wholeness presented by living organisms and human cultures. Of 
course, it is in no way attempted to give an exhaustive treatment of 
these topics, but only to indicate how the problems present them- 
selves -against the background of the general lesson of atomic physics. 

The fourth article deals with the discussion among physicists of 
the epistemological problems raised by quantum physics. Owing to 
the character of the topic, some reference to the mathematical tools 
has been unavoidable, but the understanding of the arguments de- 
mands no special knowledge. The debate led to a clarification of 
the new aspects of the observational problem, implied by the circum- 
stance that the interaction between atomic objects and measuring in- 
struments forms an integral part of quantum phenomena. Therefore, 
evidence gained by different experimental arrangements cannot be 
comprehended on accustomed lines, and the necessity of taking into 
account the conditions under which experience is obtained calls di- 
rectly for the complementary mode of description. 

The last group of articles is closely related to the first, but it is hoped 
that the improved terminology used to present the situation in quan- 
tum physics has made the general argument more easily accessible. 
In its application to problems of broader scope, emphasis is laid espe- 
cially on the presuppositions for unambiguous use of the concepts 
employed in the account of experience. The gist of the argument is 
that for objective description and harmonious comprehension it is 
necessary in almost every field of knowledge to pay attention to the 
circumstances under which evidence is obtained. 

Light and Life 

s a physicist whose studies are limited to the properties of inani- 
mate bodies, it is not without hesitation that I have accepted 
the kind invitation to address this assembly of scientists met together 
to forward our knowledge of the beneficial effects of light in the 
cure of diseases. Unable as I am to contribute to this beautiful branch 
of science that is so important for the welfare of mankind, I could at 
most comment on the purely inorganic light phenomena which have 
exerted a special attraction on physicists throughout the ages not 
least owing to the fact that light is our principal tool of observation. 
I have thought, however, that on this occasion it might perhaps be 
of interest in such a comment to enter on the problem of how far 
the results reached in the more limited domain of physics may influ- 
ence our views as regards the position of living organisms within the 
general edifice of natural science. Notwithstanding the subtle char- 
acter of the riddles of life, this problem has presented itself at every 
stage of science, the very essence of scientific explanation being the 
analysis of more complex phenomena into simpler ones. At the mo- 
ment it is the essential limitation of the mechanical description of 
natural phenomena revealed by the recent development of atomic 



theory which has lent new interest to the old problem. This de- 
velopment originated just in the closer study of the interaction be- 
tween light and material bodies which presents features that defeat 
certain demands hitherto considered as indispensable in a physical 
explanation. As I shall endeavour to show, the efforts of physicists 
to master this situation resemble in some way the attitude towards the 
aspects of life always taken more or less intuitively by biologists. 
Still, I wish to stress at once that it is only in this formal respect that 
light, which is perhaps the least complex of all physical phenomena, 
exhibits an analogy to life which shows a diversity beyond the grasp 
of scientific analysis. 

From a physical standpoint, light may be defined as transmission 
of energy between material bodies at a distance. As is well known, 
such effects find a simple explanation within the electromagnetic 
theory which may be regarded as a rational extension of classical 
mechanics suited to alleviate the contrast between action at a distance 
and at contact. According to this theory, light is described as coupled 
electric and magnetic oscillations differing from ordinary electromag- 
netic waves of radio transmission only by the greater frequency of vi- 
bration and the smaller wave-length. In fact, the practically rectilinear 
propagation of light, on which rests the location of bodies by direct 
vision or by suitable optical instruments, depends entirely on the 
smallness of the wave-length compared with the dimensions of the 
bodies concerned and of the instruments. At the same time, the 
wave character of light propagation not only forms the basis for our 
account of colour phenomena, which in spectroscopy have yielded 
such important information of the constitution of material bodies, 
but is also essential for every refined analysis of optical phenomena. 
As a typical example, I need only mention the interference patterns 
which appear when light from one source can travel to a screen 
along two different paths. Here we find that the effects which would 
be produced by the separate light beams are strengthened at such 
points of the screen where the phases of the two wave trains coincide, 
that is, where the electric and magnetic oscillations in the two beams 
have the same directions, while the effects are weakened and may 
even disappear at points where these oscillations have opposite di- 
rections and where the wave trains are said to be out of phase with 
one another. These interference patterns offer so thorough a test of 
the wave picture of light propagation that this picture cannot be con- 
sidered as a hypothesis in the usual sense of this word, but may rather 
be regarded as the adequate account of the phenomena observed. 


Still, as you all know, the problem of the nature of light has been 
subjected to renewed discussion in recent years, on account of the 
discovery of an essential feature of atomicity in the mechanism of 
energy transmission which is quite unintelligible from the point of 
view of the electromagnetic theory. In fact, any energy transfer by 
light can be traced down to individual processes in each of which a 
so-called light quantum is exchanged whose energy is equal to the 
product of the frequency of the electromagnetic oscillations and the 
universal quantum of action or Planck's constant. The obvious con- 
trast between this atomicity of the light effect and the continuity of 
the energy transfer in the electromagnetic theory presents us with a 
dilemma of a character hitherto unknown in physics. Thus, in spite 
of its obvious insufficiency, there can be no question of replacing the 
wave picture of light propagation by some other picture leaning on 
ordinary mechanical ideas. Especially, it should be emphasized that 
light quanta cannot be regarded as particles to which a well-defined 
path in the sense of ordinary mechanics can be ascribed. Just as an 
interference pattern would completely disappear if, in order to make 
sure that the light energy travelled only along one of the two paths 
between the source and the screen, we would stop one of the beams 
by a non-transparent body, so is it impossible in any phenomenon for 
which the wave constitution of light is essential to trace the path of 
the individual light quanta without essentially disturbing the phenom- 
enon under investigation. Indeed, the spatial continuity of our pic- 
ture of light propagation and the atomicity of the light effects are 
complementary aspects in the sense that they account for equally 
important features of the light phenomena which can never be 
brought into direct contradiction with one another, since their closer 
analysis in mechanical terms demand mutually exclusive experimental 
arrangements. At the same time, this very situation forces us to re- 
nounce on a complete causal account of the light phenomena and to 
be content with probability laws based on the fact that the electro- 
magnetic description of energy transfer remains valid in a statistical 
sense. This forms a typical application of the so-called correspond- 
ence argument which expresses the endeavour of utilizing to the 
outmost extent the concepts of the classical theories of mechanics and 
electrodynamics, in spite of the contrast between these theories and 
the quantum of action. 

At first, this situation may appear very uncomfortable but, as has 
often happened in science when new discoveries have led to the recog- 
nition of an essential limitation of concepts hitherto considered as 
indispensable, we are rewarded by getting a wider view and a greater 


power to correlate phenomena which before might even have ap- 
peared as contradictory. Indeed, the limitation of classical mechanics 
symbolized by the quantum of action has offered a clue to our under- 
standing of the intrinsic stability of atoms on which the mechanical 
description of natural phenomena is essentially based. Of course, it 
has always been a fundamental feature of the atomic theory that the 
indivisibility of the atoms cannot be understood in mechanical terms, 
and this situation remained practically unchanged even after the in- 
divisibility of atoms was replaced by that of the elementary electric 
particles, electrons and protons, of which atoms and molecules are 
built up. What I am referring to is not the problem of the intrinsic 
stability of these elementary particles but that of the atomic structures 
composed of them. If we attack this problem from the point of view 
of mechanics or of the electromagnetic theory, we find no sufficient 
basis on which to account for the specific properties of the elements 
and not even for the existence of rigid bodies on which all measure- 
ments used for ordering phenomena in space and time ultimately rest. 
These difficulties are now overcome by the recognition that any well- 
defined change of an atom is an individual process consisting in a 
complete transition of the atom from one of its so-called stationary 
states to another. Moreover, since just one light quantum is exchanged 
in a transition process by which light is emitted or absorbed by an 
atom, we are able by means of spectroscopic observations to measure 
directly the energy of each of these stationary states. The informa- 
tion thus derived has also been most instructively corroborated by the 
study of the energy exchanges which take place in atomic collisions 
and in chemical reactions. 

In recent years a remarkable development of atomic mechanics 
along the lines of the correspondence argument has taken place, afford- 
ing us with proper methods of calculating the energies of the station- 
ary states of atoms and the probabilities of transition processes, thus 
making our account of atomic properties as comprehensive as the 
coordination of astronomical experience by Newtonian mechanics. 
Notwithstanding the greater complexity of the general problems of 
atomic mechanics, the lesson taught us by the analysis of the simpler 
light effects has been most important for this development. Thus, 
the unambiguous use of the concept of stationary states stands in a 
similar relation of complementarity to a mechanical analysis of intra- 
atomic motions as do light quanta to the electromagnetic theory of 
radiation. Indeed, any attempt to trace the detailed course of a 
transition process would involve an uncontrollable exchange of energy 
between the atom and the measuring instruments which would com- 


pletely disturb the very energy balance we set out to investigate. 
The causal mechanical coordination of experience can be accom- 
plished only in cases where the action involved is large compared 
with the quantum and where, therefore, a subdivision of the phenom- 
ena is possible. If this condition is not fulfilled, the action of the 
measuring instruments on the object under investigation cannot be 
disregarded and will entail a mutual exclusion of the various kinds of 
information required for a complete mechanical description of the 
usual type. This apparent incompleteness of the mechanical analysis 
of atomic phenomena issues ultimately from the ignorance of the 
reaction of the object on the measuring instruments inherent in any 
measurement. Just as the general concept of relativity expresses the 
essential dependence of any phenomenon on the frame of reference 
used for its coordination in space and time, the notion of complemen- 
tarity serves to symbolize the fundamental limitation, met with in 
atomic physics, of the objective existence of phenomena independent 
of the means of their observation. 

This revision of the foundations of mechanics, extending to the 
very idea of physical explanation, not only is essential for the full 
appreciation of the situation in atomic theory but also creates a new 
background for the discussion of the problems of life in their relation 
to physics. In no way does this mean that in atomic phenomena we 
meet with features which show a closer resemblance to the properties 
of living organisms than do ordinary physical effects. At first sight, 
the essentially statistical character of atomic mechanics might even 
seem to conflict with the marvellously refined organisation of living 
beings. We must keep in mind, however, that just this complemen- 
tary mode of description leaves room for regularities in atomic proc- 
esses foreign to mechanics but as essential for our account of the 
behaviour of living organisms as for the explanation of the specific 
properties of inorganic matter. Thus, in the carbon assimilation of 
plants, on which so largely depends also the nourishment of animals, 
we are dealing with a phenomenon for the understanding of which 
the individuality of photo-chemical processes is clearly essential. 
Likewise, the non-mechanical stability of atomic structures is mark- 
edly exhibited in the characteristic properties of such highly com- 
plicated chemical combinations as chlorophyll or hemoglobine which 
play a fundamental part in the mechanism of plant assimilation and 
animal respiration. Still, analogies from ordinary chemical experi- 
ence, like the ancient comparison of life with fire, will of course yield 
no more satisfactory explanation of living organisms than will their 
resemblance with such purely mechanical contrivances as a clock- 


work. Indeed, the essential characteristics of living beings must be 
sought in a peculiar organisation in which features that may be 
analyzed by usual mechanics are interwoven with typically atomistic 
features to an extent unparalleled in inanimate matter. 

An instructive illustration of the degree to which this organisation 
is developed is exhibited by the construction and function of the eye, 
for the exploration of which the simplicity of light phenomena have 
again been most helpful. I need not here go into details but shall just 
remind you how ophthalmology has revealed to us the ideal properties 
of the human eye as an optical instrument. Indeed, the limit imposed 
on the image formation by the unavoidable interference effects coin- 
cides practically with the size of such partitions of the retina which 
have separate nervous connection with the brain. Moreover, since 
the absorption of a single light quantum by each of these retinal 
partitions is sufficient for a sight impression, the sensitiveness of the 
eye may be said to have reached the limit set by the atomic character 
of the light processes. The efficiency of the eye in both of these 
respects is actually the same as that obtained in a good telescope or 
microscope connected with a suitable amplifier so as to make the 
individual processes observable. It is true that it is possible by such 
instruments to essentially increase our powers of observation, but, 
due to the limits imposed by the fundamental properties of the light 
phenomena, no instrument is imaginable which is more efficient for 
its purpose than the eye. Now, this ideal refinement of the eye, recog- 
nized through the recent development of physics, suggests that also 
other organs, whether they serve for the reception of information 
from the surroundings or for the reaction to sense impressions, will 
exhibit a similar adaptation to their purpose, and that also here the 
feature of individuality symbolized by the quantum of action is of 
decisive importance in connection with some amplifying mechanism. 
That it has been possible to trace this limit in the eye but not, so far, 
in any other organ is due simply to the extreme simplicity of the 
light phenomena to which we have referred before. 

The recognition of the essential importance of atomistic features 
in the mechanism of living organisms is in no way sufficient, however, 
for a comprehensive explanation of biological phenomena. The ques- 
tion at issue, therefore, is whether some fundamental traits are still 
missing in the analysis of natural phenomena before we can reach 
an understanding of life on the basis of physical experience. Not- 
withstanding the fact that the multifarious biological phenomena are 
practically inexhaustible, an answer to this question can hardly be 
given without an examination of the meaning to be given to physical 


explanation still more penetrating than that to which the discovery 
of the quantum of action has already forced us. On the one hand, 
the wonderful features which are constantly revealed in physiological 
investigations and which differ so markedly from what is known of 
inorganic matter have lead biologists to the belief that no proper 
understanding of the essential aspects of life is possible in purely 
physical terms. On the other hand, the view known as vitalism can 
hardly be given an unambiguous expression by the assumption that a 
peculiar vital force, unknown to physics, governs all organic life. 
Indeed, I think we all agree with Newton that the ultimate basis of 
science is the expectation that nature will exhibit the same effects 
under the same conditions. If, therefore, we were able to push the 
analysis of the mechanism of living organisms as far as that of atomic 
phenomena, we should not expect to find any features foreign to 
inorganic matter. In this dilemma it must be kept in mind, however, 
that the conditions in biological and physical research are not di- 
rectly comparable, since the necessity of keeping the object of in- 
vestigation alive imposes a restriction on the former which finds no 
counterpart in the latter. Thus, we should doubtless kill an animal 
if we tried to carry the investigation of its organs so far that we 
could tell the part played by the single atoms in vital functions. In 
every experiment on living organisms there must remain some un- 
certainty as regards the physical conditions to which they are sub- 
jected, and the idea suggests itself that the minimal freedom we must 
allow the organism will be just large enough to permit it, so to say, 
to hide its ultimate secrets from us. On this view, the very existence 
of life must in biology be considered as an elementary fact, just as 
in atomic physics the existence of the quantum of action has to be 
taken as a basic fact that cannot be derived from ordinary mechanical 
physics. Indeed, the essential non-analyzability of atomic stability 
in mechanical terms presents a close analogy to the impossibility of 
a physical or chemical explanation of the peculiar functions char- 
acteristic of life. 

In tracing this analogy, however, we must remember that the 
problems present essentially different aspects in atomic physics and 
in biology. While in the former field we are primarily interested in 
the behaviour of matter in its simplest forms, the complexity of the 
material systems with which we are concerned in biology is of a 
fundamental nature, since even the most primitive organisms contain 
large numbers of atoms. It is true that the wide field of application 
of ordinary mechanics, including our account of the measuring instru- 
ments used in atomic physics, rests just on the possibility of largely 



disregarding the complementarity of the description entailed by the 
quantum of action in cases where we are dealing with bodies con- 
taining a great number of atoms. Notwithstanding the essential im- 
portance of the atomistic features, it is typical of biological research, 
however, that we can never control the external conditions to which 
any separate atom is subjected to the extent possible in the funda- 
mental experiments of atomic physics. In fact, we cannot even tell 
which particular atoms really belong to a living organism, since any 
vital function is accompanied by an exchange of material through 
which atoms are constantly taken up into and expelled from the or- 
ganisation which constitutes the living being. Indeed, this exchange 
of matter extends to all parts of a living organism to a degree which 
prevents a sharp distinction on an atomic scale between those features 
of its mechanism which can be unambiguously accounted for on usual 
mechanics and those for which a regard of the quantum of action is 
decisive. This fundamental difference between physical and biologi- 
cal research implies that no well-defined limit can be drawn for the 
applicability of physical ideas to the problems of life which corre- 
sponds to the distinction between the field of causal mechanical 
description and proper quantum phenomena in atomic mechanics. 
This apparent limitation of the analogy in question is rooted in the 
very definitions of the words life and mechanics which are ultimately 
a matter of convenience. On the one hand, the question of a limita- 
tion of physics in biology would lose any meaning if, instead of dis- 
tinguishing between living organisms and inanimate bodies, we ex- 
tended the idea of life to all natural phenomena. On the other hand, 
if, in accordance with common language, we were to reserve the word 
mechanics for the unambiguous causal description of natural phe- 
nomena, such a term as atomic mechanics would become meaning- 
less. I shall not enter further into such purely terminological points 
but only add that the essence of the analogy being considered is the 
obvious exclusiveness between such typical aspects of life as the self- 
preservation and the self-generation of individuals, on the one hand, 
and the subdivision necessary for any physical analysis on the other 
hand. Owing to this essential feature of complementarity, the con- 
cept of purpose, which is foreign to mechanical analysis, finds a cer- 
tain field of application in biology. Indeed, in this sense teleological 
argumentation may be regarded as a legitimate feature of physiological 
description which takes due regard to the characteristics of life in a 
way analogous to the recognition of the quantum of action in the cor- 
respondence argument of atomic physics. 



In discussing the applicability of purely physical ideas to living 
organisms we have, of course, treated life just as any other phenome- 
non of the material world. I need hardly emphasize, however, that 
this attitude, which is characteristic of biological research, involves 
no disregard of the psychological aspect of life. On the contrary, 
the recognition of the limitation of mechanical concepts in atomic 
physics would rather seem suited to conciliate the apparently con- 
trasting viewpoints of physiology and psychology. Indeed, the neces- 
sity of considering the interaction between the measuring instru- 
ments and the object under investigation in atomic mechanics exhibits 
a close analogy to the peculiar difficulties in psychological analysis 
arising from the fact that the mental content is invariably altered 
when the attention is concentrated on any special feature of it. It 
will carry us too far from our subject to enlarge upon this analogy 
which offers an essential clarification of the psycho-physical parallel- 
ism. However, I should like to emphasize that considerations of the 
kind here mentioned are entirely opposed to any attempt of seeking 
new possibilities for a spiritual influence on the behaviour of matter 
in the statistical description of atomic phenomena. For instance, it 
is impossible, from our standpoint, to attach an unambiguous mean- 
ing to the view sometimes expressed that the probability of the oc- 
currence of certain atomic processes in the body might be under the 
direct influence of the will. In fact, according to the generalized 
interpretation of the psycho-physical parallelism, the freedom of the 
will is to be considered as a feature of conscious life which corre- 
sponds to functions of the organism that not only evade a causal 
mechanical description but resist even a physical analysis carried to 
the extent required for an unambiguous application of the statistical 
laws of atomic mechanics. Without entering into metaphysical 
speculations, I may perhaps add that an analysis of the very concept 
of explanation would, naturally, begin and end with a renunciation 
as to explaining our own conscious activity. 

In conclusion, I need hardly emphasize that with none of my re- 
marks have I intended to express any kind of scepticism as to the 
future development of physical and biological sciences. Such scepti- 
cism would, indeed, be far from the mind of physicists at the present 
time when just the recognition of the limited character of our most 
fundamental concepts has resulted in such a remarkable development 
of our science. Nor has the renunciation of an explanation of life im- 
peded the wonderful progress which has taken place in all branches 
of biology, including those which have proved so beneficial in the art 
of medicine. Even if we cannot make a sharp physical distinction 


between health and disease, there is surely no room for scepticism in 
the special field which is the subject of this congress, as long as one 
does not leave the highroad of progress which has been followed with 
so great success since the pioneer work of Finsen, and the distinguish- 
ing mark of which is the most intimate combination of the investi- 
gation of the medical effects of light therapy with the study of 
its physical aspects. 



Atomic Physics 


The immortal work of Galvani which inaugurated a new epoch 
in the whole field of science is a most brilliant illustration of the 
extreme fruitfulness of an intimate combination of the exploration of 
the laws of inanimate nature with the study of the properties of living 
organisms. At this occasion, it may therefore be appropriate to review 
the attitude which scientists through the ages have taken to the ques- 
tion of the relationship beween physics and biology and especially to 
discuss the outlook created in this respect by the extraordinary de- 
velopment of atomic theory in recent time. 

From the very dawn of science, atomic theory has indeed been 
at the focus of interest in connection with the efforts to attain a 
comprehensive view of the great variety of natural phenomena. Thus 
already Democritus, who with so deep intuition emphasized the 
necessity of atomism for any rational account of the ordinary prop- 
erties of matter, attempted, as is well known, also to utilize atomistic 
ideas for the explanation of the peculiarities of organic life and even 
of human psychology. In view of the fantastic character of such 
extreme materialistic conceptions, it was a natural reaction when 
Aristotle, with his masterly comprehension of the knowledge of his 




time in physics as well as in biology, rejected atomic theory entirely 
and tried to provide a sufficiently broad frame for an account of the 
wealth of natural phenomena on the basis of essentially teleological 
ideas. The exaggeration of the Aristotelian doctrine, on its side, was, 
however, clearly brought to light by the gradual recognition of ele- 
mentary laws of nature valid as well for inanimate bodies as for living 

When thinking of the establishment of the principles of mechanics, 
which were to become the very foundations of physical science, it is, 
in this connection, not without interest to realize that Archimedes' 
discovery of the principle of equilibrium of floating objects, which, 
according to a familiar tradition, was suggested to him by the sensa- 
tion of uplift of his own body in a bath tub, might just as well have 
been based on common experience regarding the loss of weight of 
stones in water. Likewise it is to be regarded as quite accidental that 
Galileo was led to the recognition of the fundamental laws of dy- 
namics by observing the pendulum motion of a chandelier in the 
beautiful cathedral of Pisa, and not by looking at a child in a swing. 
Yet such purely external analogies were, of course, only of little 
weight for the growing appreciation of the essential unity of the 
principles governing natural phenomena, as compared to the deep- 
rooted similarities between living organisms and technical machinery 
that were disclosed by the studies of anatomy and physiology, pur- 
sued so intensely at the time of the Renaissance especially here in 

The enthusiasm for the prospects opened by the success of the 
new experimental approach to natural philosophy— encouraged in 
equal manner by the widening of the world picture due to the vision 
of Copernicus and by the elucidation of circulation mechanisms in 
animal bodies, initiated by Harvey's great achievement— found per- 
haps its most striking expression in the work of Borelli, who suc- 
ceeded to clarify in so fine detail the mechanical function of skeleton 
and muscles in animal motion. The classical character of this work 
is in no way impeded by attempts of Borelli himself and his followers 
also to explain nervous action and glandular secretion by means of 
primitive mechanical models, the obvious arbitrariness and coarseness 
of which soon gave rise to general criticism, still remembered by the 
semi-ironical name of "iatro-physicists" attached to the Borellian 
school. Likewise the endeavours, sound in their root, to apply the 
growing knowledge of typically chemical transformations of matter 
to physiological processes, which found so enthusiastic an exponent 
in Sylvius, rapidly led, by exaggerations of superficial resemblances 


of digestion and fermentation with the simplest inorganic reactions 
and their rash application to medical purposes, to an opposition ■which 
has found its expression in the labelling of such premature endeavours 
as "iatro-chemistry." 

To us the reasons for the shortcomings of these pioneer efforts 
to utilize physics and chemistry for a comprehensive explanation of 
the properties of living organisms are evident. Not only had one to 
wait until Lavoisier's time for the disclosure of the elementary prin- 
ciples of chemistry, which were to give the clue to the understanding 
of respiration and later to provide the basis for the extraordinary 
development of so-called organic chemistry, but, before Galvani's 
discoveries, a whole fundamental aspect of the laws of physics lay 
still hidden. It is most suggestive to think that the germ which, in 
the hands of Volta, Oersted, Faraday, and Maxwell, was to develop 
into a structure rivalling Newtonian mechanics in importance grew 
out of researches with a biological aim. In fact, it is difficult to 
imagine that the progress from experiments with electrically charged 
bodies, however fruitful in Franklin's hands, to the study of galvanic 
currents could have been achieved if the sensitive instruments neces- 
sary for the detection of such currents, afterwards so readily con- 
structed, had not been provided by nature itself in the nervous fabric 
of higher animals. 

It is impossible here to sketch, even in outline, the tremendous 
development of physics and chemistry since the days of Galvani, or 
to enumerate the discoveries in all branches of biology in the last 
century. We need only recall the lines leading from the pioneer 
work, in this venerable university, of Malpighi and Spallanzani to 
modern embryology and bacteriology respectively, or from Galvani 
himself to the recent fascinating researches on nerve impulses. In 
spite of the far-reaching understanding, thus obtained, of the physi- 
cal and chemical aspect of many typical biological reactions, the 
marvellous fineness of structure of the organisms and their wealth of 
interconnected regulation mechanisms go still so far beyond any 
experience about inanimate nature that we feel as removed as ever 
from an explanation of life itself on such lines. Indeed, when we 
witness the passionate scientific controversies as regards the bearing 
on this problem of the recent discoveries of poisoning effects and 
generative properties of so-called virus, we find ourselves presented 
with a dilemma just as acute as that with which Democritus and Aris- 
totle were confronted. 

In this situation it is again upon atomic theory that interest is 
concentrated, although against a very different background. Not only 



has this theory, since Dalton applied with such decisive success atom- 
istic conceptions to the elucidation of the quantitative laws govern- 
ing the constitution of chemical compounds, become the indispensable 
foundation and never-failing guide of all reasoning in chemistry; but 
the wonderful refinement of experimental technique in physics has 
even given us the means of studying phenomena which directly de- 
pend on the action of individual atoms. At the same time that this 
development has thus removed the last traces of the traditional preju- 
dice that, due to the coarseness of our senses, any proof of the actual 
existence of atoms would forever remain beyond the reach of human 
experience, it has revealed still deeper features of atomicity in the 
laws of nature than those expressed by the old doctrine of the limited 
divisibility of matter. We have indeed been taught that the very 
conceptual frame, appropriate both to give account of our experience 
in everyday life and to formulate the whole system of laws applying 
to the behaviour of matter in bulk and constituting the imposing 
edifice of so-called classical physics, had to be essentially widened if 
it was to comprehend proper atomic phenomena. In order to appre- 
ciate the possibilities which this new outlook in natural philosophy 
provides with respect to a rational attitude towards the fundamental 
problems of biology, it will, however, be necessary to recall briefly 
the principal lines of the development which has led to the elucidation 
of the situation in atomic theory. 

The starting-point of modern atomic physics was, as is well known, 
the recognition of the atomic nature of electricity itself, first indi- 
cated by Faraday's famous researches on galvanic electrolysis and 
definitely established by the isolation of the electron in the beautiful 
phenomena of electric discharges through rarefied gases, which at- 
tracted so much attention towards the end of the last century. 
While J. J. Thomson's brilliant researches soon brought to light the 
essential part played by electrons in most varied physical and chemical 
phenomena, our knowledge of the structural units of matter was, 
however, not completed until Rutherford's discovery of the atomic 
nucleus, crowning his pioneer work on the spontaneous radioactive 
transmutations of certain heavy elements. Indeed, this discovery 
offered for the first time an unquestionable explanation of the invari- 
ability of the elements in ordinary chemical reactions, in which the 
minute heavy nucleus remains unaltered, while only the distribution 
of the light electrons around it is affected. Moreover, it provides an 
immediate understanding not only of the origin of natural radioactiv- 
ity, in which we witness an explosion of the nucleus itself, but also 
of the possibility, subsequently discovered by Rutherford, of induc- 



ing transmutations of elements by bombardment with high-speed 
heavy particles which, in colliding with the nuclei, may cause their 

It would carry us too far from the subject of this address to enter 
here further upon the wonderful new field of research opened by the 
study of nuclear transmutations, which will be one of the main sub- 
jects of discussion among physicists at this meeting. The essential 
point for our argument is indeed not to be found in such new ex- 
perience but in the obvious impossibility to account for common 
physical and chemical evidence on the basis of the well-established 
main features of Rutherford's atomic model without departing radi- 
cally from the classical ideas of mechanics and electromagnetism. 
In fact, notwithstanding the insight provided by Newtonian mechanics 
into the harmony of planetary motions expressed by the Keplerian 
laws, the stability properties of mechanical models like the solar sys- 
tem which, when disturbed, have no tendency to return to their orig- 
inal state, have clearly no sufficient resemblance with the intrinsic 
stability of the electronic configurations of atoms that is responsible 
for the specific properties of the elements. Above all, this stability is 
strikingly illustrated by spectral analysis which, as is well known, has 
revealed that any element possesses a characteristic spectrum of sharp 
lines, independent of the external conditions to such an extent that it 
offers a means of identifying the material composition of even the 
most remote stars by spectroscopic observations. 

A clue to the solution of this dilemma was, however, already pro- 
vided by Planck's discovery of the elementary quantum of action 
which was the outcome of a very different line of physical research. 
As it is well known, Planck was led to this fundamental discovery by 
his ingenious analysis of such features of the thermal equilibrium 
between matter and radiation which, according to the general prin- 
ciples of thermodynamics, should be entirely independent of any 
specific properties of matter, and accordingly of any special ideas on 
atomic constitution. The existence of the elementary quantum of 
action expresses, in fact, a new trait of individuality of physical 
processes which is quite foreign to the classical laws of mechanics and 
electromagnetism and limits their validity essentially to those phe- 
nomena which involve actions large compared to the value of a single 
quantum, as given by Planck's new atomistic constant. This condi- 
tion, though amply fulfilled in the phenomena of ordinary physical 
experience, does in no way hold for the behaviour of electrons in 
atoms, and it is indeed only the existence of the quantum of action 



which prevents the fusion of the electrons and the nucleus into a 
neutral massive corpuscle of practically infinitesimal extension. 

The recognition of this situation suggested at once the description 
of the binding of each electron in the field around the nucleus as a 
succession of individual processes by which the atom is transferred 
from one of its so-called stationary states to another of these states, 
with emission of the released energy in the form of a single quantum 
of electromagnetic radiation. This view, intimately akin to Einstein's 
successful interpretation of the photoelectric effect, and borne out so 
convincingly by the beautiful researches of Franck and Hertz on 
the excitation of spectral lines by impacts of electrons on atoms, did 
in fact not only provide an immediate explanation of the puzzling 
general laws of line spectra disentangled by Balmer, Rydberg, and 
Ritz, but, with the help of spectroscopic evidence, led gradually to a 
systematic classification of the types of stationary binding of any 
electron in an atom, offering a complete explanation of the remark- 
able relationships between the physical and chemical properties of 
the elements, as expressed in the famous periodic table of Mendeleev. 
While such an interpretation of the properties of matter appeared as a 
realisation, even surpassing the dreams of the Pythagoreans, of the 
ancient ideal of reducing the formulation of the laws of nature to con- 
siderations of pure numbers, the basic assumption of the individuality 
of the atomic processes involved at the same time an essential renun- 
ciation of the detailed causal connection between physical events, 
which through the ages had been the unquestioned foundation of 
natural philosophy. 

Not only was any question of a return to a mode of description 
consistent with the principle of causality excluded by unambiguous 
experience of the most varied kind, but it soon proved possible to 
develop the original primitive attempts at accounting for the ex- 
istence of the quantum of action in atomic theory into a proper, 
essentially statistical atomic mechanics, fully comparable in consistency 
and completeness with the structure of classical mechanics of which 
it appears as a rational generalization. The establishment of this new 
so-called quantum mechanics which, as is well known, we owe above 
all to the ingenious contributions of the younger generation of physi- 
cists has, indeed, quite apart from its astounding fruitfulness in all 
branches of atomic physics and chemistry, essentially clarified the 
epistemological basis of the analysis and synthesis of atomic phe- 
nomena. The revision of the very problem of observation in this 
field, initiated by Heisenberg, one of the principal founders of quan- 
tum mechanics, has in fact led to the disclosure of hitherto disre- 



garded presuppositions for the unambiguous use of even the most 
elementary concepts on which the description of natural phenomena 
rests. The critical point is here the recognition that any attempt to 
analyse, in the customary way of classical physics, the "individuality" 
of atomic processes, as conditioned by the quantum of action, will be 
frustrated by the unavoidable interaction between the atomic objects 
concerned and the measuring instruments indispensable for that pur- 

An immediate consequence of this situation is that observations 
regarding the behaviour of atomic objects obtained with different 
experimental arrangements cannot in general be combined in the 
usual way of classical physics. In particular, any imaginable proce- 
dure aiming at the coordination in space and time of the electrons 
in an atom will unavoidably involve an essentially uncontrollable 
exchange of momentum and energy between the atom and the meas- 
uring agencies, entirely annihilating the remarkable regularities of 
atomic stability for which the quantum of action is responsible. Con- 
versely, any investigation of such regularities, the very account of 
which implies the conservation laws of energy and momentum, will 
in principle impose a renunciation as regards the space-time coordi- 
nation of the individual electrons in the atom. Far from being in- 
consistent, the aspects of quantum phenomena revealed by experi- 
ence obtained under such mutually exclusive conditions must thus be 
considered complementary in quite a novel way. The viewpoint 
of "complementarity" does, indeed, in no way mean an arbitrary 
renunciation as regards the analysis of atomic phenomena, but is on 
the contrary the expression of a rational synthesis of the wealth of 
experience in this field, which exceeds the limits to which the applica- 
tion of the concept of causality is naturally confined. 

Notwithstanding the encouragement given to the pursuit of such 
inquiries by the great example of relativity theory which, just 
through the disclosure of unsuspected presuppositions for the un- 
ambiguous use of all physical concepts, opened new possibilities for 
the comprehension of apparently irreconcilable phenomena, we must 
realize that the situation met with in modern atomic theory is entirely 
unprecedented in the history of physical science. Indeed, the whole 
conceptual structure of classical physics, brought to so wonderful a 
unification and completion by Einstein's work, rests on the assump- 
tion, well adapted to our daily experience of physical phenomena, 
that it is possible to discriminate between the behaviour of material 
objects and the question of their observation. For a parallel to the 
lesson of atomic theory regarding the limited applicability of such 



customary idealisations, we must in fact turn to quite other branches 
of science, such as psychology, or even to that kind of epistemological 
problems with which already thinkers like Buddha and Lao Tse 
have been confronted, when trying to harmonize our position as 
spectators and actors in the great drama of existence. Still, the 
recognition of an analogy in the purely logical character of the prob- 
lems which present themselves in so widely separated fields of human 
interest does in no way imply acceptapce in atomic physics of any 
mysticism foreign to the true spirit of science, but on the contrary 
it gives us an incitation to examine whether the straightforward solu- 
tion of the unexpected paradoxes met with in the application of our 
simplest concepts to atomic phenomena might not help us to clarify 
conceptual difficulties in other domains of experience. 

There has also been no lack of suggestions to look for a direct 
correlation between life or free will and those features of atomic 
phenomena for the comprehension of which the frame of classical 
physics is obviously too narrow. In fact, it is possible to point out 
many characteristic features of the reactions of living organisms, 
like the sensitivity of visual perception or the induction of gene 
mutation by penetrating radiation, which undoubtedly involve an 
amplification of the effects of individual atomic processes, similar to 
that on which the experimental technique of atomic physics is essen- 
tially based. Still, the recognition that the fineness of organization 
and regulation mechanisms of living beings goes even so far beyond 
any previous expectation does in itself in no way enable us to account 
for the peculiar characteristics of life. Indeed, the so-called holistic 
and finalistic aspects of biological phenomena can certainly not be 
immediately explained by the feature of individuality of atomic 
processes disclosed by the discovery of the quantum of action; rather 
would the essentially statistical character of quantum mechanics at 
first sight seem even to increase the difficulties of understanding the 
proper biological regularities. In this dilemma, however, the gen- 
eral lesson of atomic theory suggests that the only way to reconcile 
the laws of physics with the concepts suited for a description of the 
phenomena of life is to examine the essential difference in the condi- 
tions of the observation of physical and biological phenomena. 

First of all we must realize that every experimental arrangement 
with which we could study the behaviour of the atoms constituting 
an organism to the extent to which this can be done for single atoms 
in the fundamental experiments of atomic physics will exclude the 
possibility of maintaining the organism alive. The incessant ex- 
change of matter which is inseparably connected with life will even 



imply the impossibility of regarding an organism as a well-defined 
system of material particles like the systems considered in any account 
of the ordinary physical and chemical properties of matter. In fact, 
we are led to conceive the proper biological regularities as represent- 
ing laws of nature complementary to those appropriate to the ac- 
count of the properties of inanimate bodies, in analogy with the com- 
plementary relationship between the stability properties of the atoms 
themselves and such behaviour of their constituent particles as allows 
of a description in terms of space-time coordination. In this sense, 
the existence of life itself should be considered, both as regards its 
definition and observation, as a basic postulate of biology, not sus- 
ceptible of further analysis, in the same way as the existence of the 
quantum of action, together with the ultimate atomicity of matter, 
forms the elementary basis of atomic physics. 

It will be seen that such a viewpoint is equally removed from the 
extreme doctrines of mechanism and vitalism. On the one hand, it 
condemns as irrelevant any comparison of living organisms with ma- 
chines, be these the relatively simple constructions contemplated by 
the old iatro-physicists, or the most refined modern amplifier de- 
vices, the uncritical emphasis of which would expose us to deserve 
the nickname of "iatro-quantists." On the other hand, it rejects as 
irrational all such attempts at introducing some kind of special bio- 
logical laws inconsistent with well-established physical and chemical 
regularities, as have in our days been revived under the impression of 
the wonderful revelations of embryology regarding cell growth and 
division. In this connection it must be especially remembered that 
the possibility of avoiding any such inconsistency within the frame 
of complementarity is given by the very fact that no result of bio- 
logical investigation can be unambiguously described otherwise than 
in terms of physics and chemistry, just as any account of experience 
even in atomic physics must ultimately rest on the use of the concepts 
indispensable for a conscious recording of sense impressions. 

The last remark brings us back into the realm of psychology, 
where the difficulties presented by the problems of definition and 
observation in scientific investigations have been clearly recognized 
long before such questions became acute in natural science. Indeed, 
the impossibility in psychical experience to distinguish between the 
phenomena themselves and their conscious perception clearly de- 
mands a renunciation of a simple causal description on the model of 
classical physics, and the very way in which words like "thoughts" 
and "feelings" are used to describe such experience reminds one most 
suggestively of the complementarity encountered in atomic physics. 


I shall not here enter into any further detail but only emphasize that 
it is just this impossibility of distinguishing, in introspection, sharply 
between subject and object which provides the necessary latitude for 
the manifestation of volition. To connect free will more directly 
with limitation of causality in atomic physics, as it is often suggested, 
is, however, entirely foreign to the tendency underlying the remarks 
here made about biological problems. 

In concluding this address I hope that the temerity of a physicist 
venturing so far outside his restricted domain of science may be for- 
given in view of the most welcome opportunity of profitable discus- 
sion offered to physicists and biologists by this gathering to honour 
the memory of the great pioneer to whose fundamental discoveries 
both branches of science owe so much. 

Natural Philosophy 


Human Cultures 

It is only with great hesitation that I have accepted the kind invita- 
tion to address this assembly of distinguished representatives of 
the anthropological and ethnographical sciences of which I, as a 
physicist, have of course no first-hand knowledge. Still, on this 
special occasion when even the historical surroundings speak to every 
one of us about aspects of life other than those discussed at the regular 
congress proceedings, it might perhaps be of interest to try with a 
few words to' draw your attention to the epistemological aspect of 
the latest development of natural philosophy and its bearing on gen- 
eral human problems. Notwithstanding the great separation be- 
tween our different branches of knowledge, the new lesson which 
has been impressed upon physicists regarding the caution with which 
all usual conventions must be applied as soon as we are not concerned 
with everyday experience may, indeed, be suited to remind us in a 
novel way of the dangers, well known to humanists, of judging from ' 
our own standpoint cultures developed within other societies. 

Of course it is impossible to distinguish sharply between natural 
philosophy and human culture. The physical sciences are, in fact, 
an integral part of our civilization, not only because our ever- 




increasing mastery of the forces of nature has so completely changed 
the material conditions of life, but also because the study of these 
sciences has contributed so much to clarify the background of our 
own existence. What has it not meant in this respect that we no 
more consider ourselves to be privileged in living at the centre of 
the universe, surrounded by less fortunate societies inhabiting the 
edges of the abyss, but, through the development of astronomy and 
geography, have realized that we all share a small spherical planet 
of the solar system which again is only a small part of still larger 
systems. How forceful an admonition about the relativity of all 
human judgments have we not also in our days received through 
the renewed revision of the presuppositions underlying the unam- 
biguous use of even our most elementary concepts such as space 
and time, which, in disclosing the essential dependence of every physi- 
cal phenomenon on the standpoint of the observer, has contributed 
so largely to the unity and beauty of our whole world-picture. 

While the importance of these great achievements for our general 
outlook is commonly realized, it is hardly yet so as regards the un- 
suspected epistemological lesson which the opening of quite new 
realms of physical research has given us in the latest years. Our 
penetration into the world of atoms, hitherto closed to the eyes of 
man, is indeed an adventure which may be compared with the great 
journeys of discovery of the circumnavigators and the bold explora- 
tions of astronomers into the depths of celestial space. As is well 
known, the marvellous development of the art of physical experi- 
mentation not only has removed the last traces of the old belief that 
the coarseness of our senses would forever prevent us from obtaining 
direct information about individual atoms, but has even shown us 
that the atoms themselves consist of still smaller corpuscles which can 
be isolated and the properties of which can be investigated sepa- 
rately. At the same time we have, however, in this fascinating field 
of experience been taught that the laws of nature hitherto known, 
which constitute the grand edifice of classical physics, are valid only 
when we deal with bodies consisting of practically infinite numbers 
of atoms. The new knowledge concerning the behaviour of single 
atoms and atomic corpuscles has, in fact, revealed an unexpected 
limit for the subdivision of all physical actions extending far beyond 
the old doctrine of the limited divisibility of matter and giving every 
atomic process a peculiar individual character. This discovery has, 
in fact, yielded a quite new basis for the understanding of the in- 
trinsic stability of atomic structures, which, in the last resort, condi- 
tions the regularities of all ordinary experience. 


How radical a change in our attitude towards the description of 
nature this development of atomic physics has brought about is per- 
haps most clearly illustrated by the fact that even the principle of 
causality, so far regarded as the unquestioned foundation for all 
interpretation of natural phenomena, has proved too narrow a frame 
to embrace the peculiar regularities governing individual atomic 
processes. Certainly everyone will understand that physicists have 
needed very cogent reasons to renounce the ideal of causality itself; 
but in the study of atomic phenomena we have repeatedly been 
taught that questions which were believed to have received long ago 
their final answers had most unexpected surprises in store for us. 
You will surely all have heard about the riddles regarding the most 
elementary properties of light and matter which have puzzled physi- 
cists so much in recent years. The apparent contradictions which we 
have met in this respect are, in fact, as acute as those which gave rise 
to the development of the theory of relativity in the beginning of 
this century and have, just as the latter, only found their explanation 
by a closer examination of the limitation imposed by the new experi- 
ences themselves on the unambiguous use of the concepts entering 
into the description of the phenomena. While in relativity theory the 
decisive point was the recognition of the essentially different ways in 
which observers moving relatively to each other will describe the 
behaviour of given objects, the elucidation of the paradoxes of atomic 
physics has disclosed the fact that the unavoidable interaction between' 
the objects and the measuring instruments sets an absolute limit to 
the possibility of speaking of a behaviour of atomic objects which is 
independent of the means of observation. 

We are here faced with an epistemological problem quite new in 
natural philosophy, where all description of experiences has so far 
been based upon the assumption, already inherent in ordinary con- 
ventions of language, that it is possible to distinguish sharply between 
the behaviour of objects and the means of observation. This assump- 
tion is not only fully justified by all everyday experience but even 
constitutes the whole basis of classical physics, which, just through 
the theory of relativity, has received such a wonderful completion. 
As soon as we are dealing, however, with phenomena like individual 
atomic processes which, due to their very nature, are essentially de- 
termined by the interaction between the objects in question and the 
measuring instruments necessary for the definition of the experimental 
arrangements, we are, therefore, forced to examine more closely the 
question of what kind of knowledge can be obtained concerning the 
objects. In this respect we must, on the one hand, realize that the 



aim of every physical experiment— to gain knowledge under repro- 
ducible and communicable conditions— leaves us no choice but to use 
everyday concepts, perhaps refined by the terminology of classical 
physics, not only in all accounts of the construction and manipulation 
of the measuring instruments but also in the description of the actual 
experimental results. On the other hand, it is equally important to 
understand that just this circumstance implies that no result of an 
experiment concerning a phenomenon which, in principle, lies out- 
side the range of classical physics can be interpreted as giving in- 
formation about independent properties of the objects, but is inher- 
ently connected with a definite situation in the description of which 
the measuring instruments interacting with the objects also enter 
essentially. This last fact gives the straightforward explanation of 
the apparent contradictions which appear when results about atomic 
objects obtained by different experimental arrangements are tenta- 
tively combined into a self-contained picture of the object. 

Information regarding the behaviour of an atomic object obtained 
under definite experimental conditions may, however, according to a 
terminology often used in atomic physics, be adequately character- 
ized as complementary to any information about the same object 
obtained by some other experimental arrangement excluding the ful- 
filment of the first conditions. Although such kinds of information 
cannot be combined into a single picture by means of ordinary con- 
cepts, they represent indeed equally essential aspects of any knowl- 
edge of the object in question which can be obtained in this domain. 
The recognition of such a complementary character of the mechanical 
analogies by which one has attempted to visualize the individual 
radiative effects has, in fact, led to an entirely satisfactory solution 
of the riddles of the properties of light alluded to above. In the same 
way it is only by taking into consideration the complementary rela- 
tionship between the different experiences concerning the behaviour 
of atomic corpuscles that it has been possible to obtain a clue to the 
understanding of the striking contrast between the properties of ordi- 
nary mechanical models and the peculiar laws of stability governing 
atomic structures which form the basis for every closer explanation 
of the specific physical and chemical properties of matter. 

Of course I have no intention, on this occasion, of entering more 
closely into such details, but I hope that I have been able to give you 
a sufficiently clear impression of the fact that we are here in no way 
concerned with an arbitrary renunciation as regards the detailed anal- 
ysis of the almost overwhelming richness of our rapidly increasing 
experience in the realm of atoms. On the contrary, we have to do 



with a rational development of our means of classifying and compre- 
hending new experience which, due to its very character, finds no 
place within the frame of causal description that is only suited to 
account for the behaviour of objects as long as this behaviour is in- 
dependent of the means of observation. Far from containing any 
mysticism contrary to the spirit of science, the viewpoint of com- 
plementarity forms indeed a consistent generalization of the ideal of 

However unexpected this development may appear in the domain 
of physics, I am sure that many of you will have recognized the close 
analogy between the situation as regards the analysis of atomic phe- 
nomena, which I have described, and characteristic features of the 
problem of observation in human psychology. Indeed, we may say 
that the trend of modern psychology can be characterized as a reac- 
tion against the attempt at analyzing psychical experience into ele- 
ments which can be associated in the same way as are the results of 
measurements in classical physics. In introspection it is clearly im- 
possible to distinguish sharply between the phenomena themselves 
and their conscious perception, and although we may often speak of 
lending our attention to some particular aspect of a psychical experi- 
ence, it will appear on closer examination that we really have to do, 
in such cases, with mutually exclusive situations. We all know the 
old saying that, if we try to analyze our own emotions, we hardly 
possess them any longer, and in that sense we recognize between 
psychical experiences, for the description of which words such as 
"thoughts" and "feelings" are adequately used, a complementary re- 
lationship similar to that between the experiences regarding the be- 
haviour of atoms obtained under different experimental arrangements 
and described by means of different analogies taken from our usual 
ideas. By such a comparison it is, of course, in no way intended to 
suggest any closer relation between atomic physics and psychology, 
but merely to stress an epistemological argument common to both 
fields, and thus to encourage us to see how far the solution of the 
relatively simple physical problems may be helpful in clarifying the 
more intricate psychological questions with which human life con- 
fronts us, and which anthropologists and ethnologists so often meet 
in their investigations. 

Coming now closer to our subject of the bearing of such viewpoints 
on the comparison of different human cultures, we shall first stress 
the typical complementary relationship between the modes of be- 
haviour of living beings characterized by the words "instinct" and 
"reason." It is true that any such words are used in very different 


senses; thus, instinct may mean motive power or inherited behaviour, 
and reason may denote deeper sense as •well as conscious argumenta- 
tion. What we are concerned with is, however, only the practical 
way in which these words are used to discriminate between the 
different situations in which animals and men find themselves. Of 
course, nobody will deny our belonging to the animal world, and it 
would even be very difficult to find an exhaustive definition charac- 
terizing man among the other animals. Indeed, the latent possibilities 
in any living organism are not easily estimated, and I think that there 
is none of us who has not sometimes been deeply impressed by the 
extent to which circus animals can be drilled. Not even with respect 
to the conveyance of information from one individual to another 
would it be possible to draw a sharp separation between animals and 
man; but of course our power of speech places us in this respect in an 
essentially different situation, not only in the exchange of practical 
experience, but first of all in the possibility of transmitting to children, 
through education, the traditions concerning behaviour and reasoning 
which form the basis of any human culture. 

As regards reason compared with instinct, it is, above all, essential 
to realize that no proper human thinking is imaginable without the 
use of concepts framed in some language which every generation has 
to learn anew. This use of concepts, in fact, not only is to a large 
extent suppressing instinctive life, but stands even largely in an ex- 
clusive relationship of complementarity to the display of inherited 
instincts. The astonishing superiority of lower animals compared 
with man in utilizing the possibilities of nature for the maintenance 
and propagation of life has certainly often its true explanation in the 
fact that on the part of such animals we can detect no conscious 
thinking, in our sense of the word. Similarly, the amazing capacity of 
so-called primitive people to orientate themselves in forests or deserts, 
which, though apparently lost in more civilized societies, may on oc- 
casion be revived in any of us, might justify the conclusion that such 
feats are only possible when no recourse is taken to conceptual 
thinking, which on its side is adapted to far more varied purposes of 
primary importance for the development of civilization. Just be- 
cause it is not yet awake to the use of concepts, a newborn child can 
hardly be reckoned as a human being; but belonging to the species 
of man, it has, of course, though more helpless a creature than most 
young animals, the organic possibilities of receiving through educa- 
tion a culture which enables it to take its place in some human 



Such considerations confront us at once with the question whether 
the widespread belief that every child is born with a predisposition 
for the adoption of a specific human culture is really well founded, or 
whether one has not rather to assume that any culture can be im- 
planted and thrive on quite different physical backgrounds. Here 
we are of course touching a subject of still unsettled controversies 
between geneticists, who pursue most interesting studies on the in- 
heritance of physical characters. In connection with such discus- 
sions, however, we must above all bear in mind that the distinction 
between the concepts genotype and phenotype, so fruitful for the 
clarification of heredity in plants and animals, essentially presupposes 
the subordinate influence of the external conditions of life on the 
characteristic properties of the species. In the case of the specific 
cultural characters of human societies the problem is, however, re- 
versed in the sense that the basis for the classification is here the tra- 
ditional habits shaped by the histories of the societies and their natural 
environments. These habits, as well as their inherent presuppositions, 
must therefore be analyzed in detail before any possible influence of 
inherited biological differences on the development and maintenance 
of the cultures concerned can be estimated. Indeed, in characteriz- 
ing different nations and even different families within a nation, we 
may to a large extent consider biological traits and spiritual traditions 
as independent of each other, and it would even be tempting to re- 
serve by definition the adjective "human" for those characters which 
are not directly bound to bodily inheritance. 

At first sight, it might perhaps appear that such an attitude would 
mean unduly stressing merely dialectic points. But the lesson which 
we have received from the whole growth of the physical sciences is 
that the germ of fruitful development often lies just in the proper 
choice of definitions. When we think, for instance, of the clarification 
brought about in various branches of science by the argumentation 
of relativity theory, we see indeed what advance may lie in such 
formal refinements. As I have already hinted at earlier in this ad- 
dress, relativistic viewpoints are certainly also helpful in promoting 
a more objective attitude as to relationships between human cultures, 
the traditional differences of which in many ways resemble the dif- 
ferent equivalent manners in which physical experience can be de- 
scribed. Still, this analogy between physical and humanistic prob- 
lems is of limited scope and its exaggeration has even led to misunder- 
standing the essence of the theory of relativity itself. In fact, the 
unity of the relativistic world picture implies precisely the pos- 
sibility for any one observer to predict within his own conceptual 


frame how any other observer will coordinate experience within the 
frame natural to him. The main obstacle to an unprejudiced atti- 
tude towards the relation between various human cultures is, how- 
ever, the deep-rooted differences of the traditional backgrounds on 
which the cultural harmony in different human societies is based and 
which exclude any simple comparison between such cultures. 

It is above all in this connection that the viewpoint of comple- 
mentarity offers itself as a means of coping with the situation. In 
fact, when studying human cultures different from our own, we have 
to deal with a particular problem of observation which on closer 
consideration shows many features in common with atomic or psy- 
chological problems, where the interaction between objects and 
measuring tools, or the inseparability of objective content and observ- 
ing subject, prevents an immediate application of the conventions 
suited to accounting for experiences of daily life. Especially in the 
study of cultures of primitive peoples, ethnologists not only are, 
indeed, aware of the risk of corrupting such cultures by the necessary 
contact, but are even confronted with the problem of the reaction 
of such studies on their own human attitude. What I here allude 
to is the experience, well known to explorers, of the shaking of their 
hitherto unrealized prejudices through the experience of the un- 
suspected inner harmony human life can present even under con- 
ventions and traditions most radically different from their own. As 
a specially drastic example I may perhaps here remind you of the 
extent to which in certain societies the roles of men and women are 
reversed, not only regarding domestic and social duties but also 
regarding behaviour and mentality. Even if many of us, in such a 
situation, might perhaps at first shrink from admitting the possibility 
that it is entirely a caprice of fate that the people concerned have 
their specific culture and not ours, and we not theirs instead of our 
own, it is clear that even the slightest suspicion in this respect implies 
a betrayal of the national complacency inherent in any human cul- 
ture resting in itself. 

Using the word much as it is used, in atomic physics, to characterize 
the relationship between experiences obtained by different experi- 
mental arrangements and visualizable only by mutually exclusive 
ideas, we may truly say that different human cultures are comple- 
mentary to each other. Indeed, each such culture represents a har- 
monious balance of traditional conventions by means of which latent 
potentialities of human life can unfold themselves in a way which 
reveals to us new aspects of its unlimited richness and variety. Of 
course, there cannot, in this domain, be any question of such abso- 


lutely exclusive relationships as those between complementary ex- 
periences about the behaviour of well-defined atomic objects, since 
hardly any culture exists which could be said to be fully self-con- 
tained. On the contrary, we all know from numerous examples how 
a more or less intimate contact between different human societies 
can lead to a gradual fusion of traditions, giving birth to a quite new 
culture. The importance in this respect of the mixing of populations 
through emigration or conquest for the advancement of human civi- 
lization need hardly be recalled. It is, indeed, perhaps the greatest 
prospect of humanistic studies to contribute through an increasing 
knowledge of the history of cultural development to that gradual 
removal of prejudices which is the common aim of all science. 

As I stressed in the beginning of this address, it is, of course, far 
beyond my capacities to contribute in any direct way to the solution 
of the problems discussed among the experts at this congress. My 
only purpose has been to give an impression of a general epistemo- 
logical attitude which we have been forced to adopt in a field as far 
from human passions as the analysis of simple physical experiments. 
I do not know, however, whether I have found the right words to 
convey to you this impression, and before I conclude I may perhaps 
be allowed to relate an experience which once most vividly reminded 
me of my deficiencies in this respect. In order to explain to an 
audience that I did not use the word prejudice to imply any con- 
demnation of other cultures, but merely to characterize our neces- 
sarily prejudiced conceptual frame, I referred jokingly to the tradi- 
tional prejudices which the Danes cherish with regard to their Swe- 
dish brothers on the other side of the beautiful Sound outside these 
windows, with whom we have fought through centuries even within 
the walls of this castle, and from contact with whom we have, 
through the ages, received so much fruitful inspiration. Now you 
will realize what a shock I got when, after my address, a member of 
the audience came up to me and said that he could not understand 
why I hated the Swedes. Obviously I must have expressed myself 
most confusingly on that occasion, and I am afraid that also to-day 
I have talked in a very obscure way. Still, I hope that I have not 
spoken so unclearly as to give rise to any such misunderstandings of 
the trend of my argument. 

Discussion with Einstein 

on Epistemological Problems 

in Atomic Physics 

When invited by the Editor of the series "Living Philos- 
ophers" to write an article for this volume in which con- 
temporary scientists are honouring the epoch-making contributions 
of Albert Einstein to the progress of natural philosophy and are 
acknowledging the indebtedness of our whole generation for the 
guidance his genius has given us, I thought much of the best way of 
explaining how much I owe to him for inspiration. In this connec- 
tion, the majoyjjccasions. through the years on which I had the privi- 
lege to discuss with Einstein epistemological problems raised by the 
modern development of atomic physics have come back vividly to 
my mind and I have felt that I could hardly attempt anything better 
than to give an account of these discussions which have been of 
greatest value and stimulus to me. I hope also that the account may 
convey to wider circles an impression of how essential the open- 
minded exchange of ideas has been for the progress in a field where 
new experience has time after time demanded a reconsideration of our 

From the very beginning the main point under debate has been 
the attitude to take to the departure from customary principles of 

3 2 



natural philosophy characteristic of the novel development of physics 
which was initiated in the first year of this century by Planck's dis- 
covery of the universal quantum of action. This discovery, which 
revealed a feature of atomicity in the laws of nature going far beyond 
the old doctrine of the limited divisibility of matter, has indeed 
taught us that the classical theories of physics are idealizations which 
can be unambiguously applied only in the limit where all actions in- 
volved are large compared with the quantum. The question at issue 
has been whether the renunciation of a causal mode of description of 
atomic processes involved in the endeavours to cope with the situa- 
tion should be regarded as a temporary departure from ideals to be 
ultimately revived or whether we are faced with an irrevocable step 
towards obtaining the proper harmony between analysis and synthesis 
of physical phenomena. To describe the background of our discus- 
sions and to bring out as clearly as possible the arguments for the 
contrasting viewpoints, I have felt it necessary to go to a certain 
length in recalling some main features of the development to which 
Einstein himself has contributed so decisively. 

As is well known, it was the intimate relation, elucidated primarily 
by Boltzmann, between the laws of thermodynamics and the statis- 
tical regularities exhibited by mechanical systems with many degrees 
of freedom, which guided Planck in his ingenious treatment of the 
problem of thermal radiation, leading him to his fundamental dis- 
covery. While, in his work, Planck was principally concerned with 
considerations of essentially statistical character and with great cau- 
tion refrained from definite conclusions as to the extent to which the 
existence of the quantum implied a departure from the foundations of 
mechanics and electrodynamics, Einstein's great original contribution 
to quantum theory (1905) was just the recognition of how physical 
phenomena like the photo-effect may depend directly on individual 
quantum effects. 1 In these very same years when, in the development 
of his theory of relativity, Einstein laid down a new foundation for 
physical science, he explored with a most daring spirit the novel fea- 
tures of atomicity which pointed beyond the framework of classical 

With unfailing intuition Einstein thus was led step by step to the 
conclusion that any radiation process involves the emission or ab- 
sorption of individual light quanta or "photons" with energy and 

E = hv and P = ha, (1) 

respectively, where h is Planck's constant, while v and a are the 

i A. Einstein, Ann. Phys., 11, 132 (1905). 




number of vibrations per unit time and the number of waves per 
unit length, respectively. Notwithstanding its fertility, the idea of 
the photon implied a quite unforeseen dilemma, since any simple 
corpuscular picture of radiation would obviously be irreconcilable 
with interference effects, which present so essential an aspect of radia- 
tive phenomena, and which can be described only in terms of a wave 
picture. The acuteness of the dilemma is stressed by the fact that 
the interference effects offer our only means of defining the concepts 
of frequency and wave-length entering into the very expressions 
for the energy and momentum of the photon. 

In this situation, there could be no question of attempting a causal 
analysis of radiative phenomena, but only, by a combined use of 
the contrasting pictures, to estimate probabilities for the occurrence 
of the individual radiation processes. However, it is most important 
to realize that the recourse to probability laws under such circum- 
stances is essentially different in aim from the familiar application of 
statistical considerations as practical means of accounting for the 
properties of mechanical systems of great structural complexity. In 
fact, in quantum physics we are presented not with intricacies of this 
kind, but with the inability of the classical frame of concepts to com- 
prise the peculiar feature of indivisibility, or "individuality," char- 
acterizing the elementary processes. 

The failure of the theories of classical physics in accounting for 
atomic phenomena was further accentuated by the progress of our 
knowledge of the structure of atoms. Above all, Rutherford's dis- 
covery of the atomic nucleus (1911) revealed at once the inade- 
quacy of classical mechanical and electromagnetic concepts to ex- 
plain the inherent stability of the atom. Here again the quantum 
theory offered a clue for the elucidation of the situation and especially 
it was found possible to account for the atomic stability, as well as 
for the empirical laws governing the spectra of the elements, by as- 
suming that any reaction of the atom resulting in a change of its 
energy involved a complete transition between two so-called sta- 
tionary quantum states and that, in particular, the spectra were 
emitted by a step-like process in which each transition is accom- 
panied by the emission of a monochromatic light quantum of an 
energy just equal to that of an Einstein photon. 

These ideas, which were soon confirmed by the experiments of 
Franck and Hertz (1914) on the excitation of spectra by impact of 
electrons on atoms, involved a further renunciation of the causal 
mode of description, since evidently the interpretation of the spectral 
laws implies that an atom in an excited state in general will have the 



possibility of transitions with photon emission to one or another of 
its lower energy states. In fact, the very idea of stationary states is 
incompatible with any directive for the choice between such transi- 
tions and leaves room only for the notion of the relative probabilities 
of the individual transition processes. The only guide in estimating 
such probabilities was the so-called correspondence principle which 
originated in the search for the closest possible connection between 
the statistical account of atomic processes and the consequences to 
be expected from classical theory, which should be valid in the limit 
where the actions involved in all stages of the analysis of the phe- 
nomena are large compared with the universal quantum. 

At that time, no general self-consistent quantum theory was yet in 
sight, but the prevailing attitude may perhaps be illustrated by the 
following passage from a lecture by the writer from 1913: 2 

I hope that I have expressed myself sufficiendy clearly so that you may 
appreciate the extent to which these considerations conflict with the ad- 
mirably consistent scheme of conceptions which has been rightly termed 
the classical theory of electrodynamics. On the other hand, I have tried 
to convey to you the impression that-just by emphasizing so strongly 
this conflict— it may also be possible in course of time to establish a cer- 
tain coherence in the new ideas. 

Important progress in the development of quantum theory was 
made by Einstein himself in his famous article on radiative equilibrium 
in 1917, 3 where he showed that Planck's law for thermal radiation 
could be simply deduced from assumptions conforming with the 
basic ideas of the quantum theory of atomic constitution. To this 
purpose, Einstein formulated general statistical rules regarding the 
occurrence of radiative transitions between stationary states, assum- 
ing not only that, when the atom is exposed to a radiation field, ab- 
sorption as well as emission processes will occur with a probability 
per unit time proportional to the intensity of the irradiation, but that 
even in the absence of external disturbances spontaneous emission 
processes will take place with a rate corresponding to a certain a priori 
probability. Regarding the latter point, Einstein emphasized the fun- 
damental character of the statistical description in a most suggestive 
way by drawing attention to the analogy beween the assumptions 
regarding the occurrence of the spontaneous radiative transitions and 
the well-known laws governing transformations of radioactive sub- 

2 N. Bohr, The Theory of Spectra and Atomic Constitution, Cambridge, 
University Press, 1922. 
8 A. Einstein, Physik. Z., IS, 121 (1917). 


In connection with a thorough examination of the exigencies of 
thermodynamics as regards radiation problems, Einstein stressed the 
dilemma still further by pointing out that the argumentation implied 
that any radiation process was "unidirected" in the sense that not 
only is a momentum corresponding to a photon with the direction of 
propagation transferred to an atom in the absorption process, but that 
also the emitting atom will receive an equivalent impulse in the op- 
posite direction, although there can on the wave picture be no ques- 
tion of a preference for a single direction in an emission process. 
Einstein's own attitude to such startling conclusions is expressed in a 
passage at the end of the article (loc. cit., p. 127f.), which may be 
translated as follows: 

These features of the elementary processes would seem to make the 
development of a proper quantum treatment of radiation almost unavoid- 
able. The weakness of the theory lies in the fact that, on the one hand, 
no closer connection with the wave concepts is obtainable and that, on the 
other hand, it leaves to chance (Zufall) the time and the direction of the 
elementary processes; nevertheless, I have full confidence in the reliability 
of the way entered upon. 

When I had the great experience of meeting Einstein for the first 
time during a visit to Berlin in 1920, these fundamental questions 
formed the theme of our conversations. The discussions, to which 
I have often reverted in my thoughts, added to all my admiration for 
Einstein a deep impression of his detached attitude. Certainly, his 
favoured use of such picturesque phrases as "ghost waves (Gespen- 
sterf elder) guiding the photons" implied no tendency to mysticism, 
but illuminated rather a profound humour behind his piercing re- 
marks. Yet, a certain difference in attitude and outlook remained, 
since, with his mastery for co-ordinating apparently contrasting ex- 
perience without abandoning continuity and causality, Einstein was 
perhaps more reluctant to renounce such ideals than someone for 
whom renunciation in this respect appeared to be the only way open 
to proceed with the immediate task of co-ordinating the multifarious 
evidence regarding atomic phenomena, which accumulated from day 
to day in the exploration of this new field of knowledge. 

In the following years, during which the atomic problems attracted 
the attention of rapidly increasing circles of physicists, the apparent 
contradictions inherent in quantum theory were felt ever more 
acutely. Illustrative of this situation is the discussion raised by the 
discovery of the Stern-Gerlach effect in 1922. On the one hand, this 



effect gave striking support to the idea of stationary states and in 
particular to the quantum theory of the Zeeman effect developed by 
Sommerfeld; on the other hand, as exposed so clearly by Einstein 
and Ehrenfest, 4 it presented with unsurmountable difficulties any 
attempt at forming a picture of the behaviour of atoms in a magnetic 
field. Similar, paradoxes were raised by the discovery by Compton 
(1924) of the change in wave-length accompanying the scattering of 
X-rays by electrons. This phenomenon afforded, as is well known, 
a most direct proof of the adequacy of Einstein's view regarding the 
transfer of energy and momentum in radiative processes; at the same 
time, it was equally clear that no simple picture of a corpuscular 
collision could offer an exhaustive description of the phenomenon. 
Under the impact of such difficulties, doubts were for a time enter- 
tained even regarding the conservation of energy and momentum in 
the individual radiation processes; 5 a view, however, which very soon 
had to be abandoned in face of more refined experiments bringing out 
the correlation between the deflection of the photon and the corre- 
sponding electron recoil. 

The way to the clarification of the situation was, indeed, first to 
be paved by the development of a more comprehensive quantum 
theory. A first step towards this goal was the recognition by de 
Broglie in 1925 that the wave-corpuscle duality was not confined to 
the properties of radiation, but was equally unavoidable in account- 
ing for the behaviour of material particles. This idea, which was 
soon convincingly confirmed by experiments on electron interfer- 
ence phenomena, was at once greeted by Einstein, who had already 
envisaged the deep-going analogy between the properties of thermal 
radiation and of gases in the so-called degenerate state. 6 The new 
line was pursued with the greatest success by Schrodinger (1926) 
who, in particular, showed how the stationary states of atomic sys- 
tems could be represented by the proper solutions of a wave-equa- 
tion to the establishment of which he was led by the formal analogy, 
originally traced by Hamilton, between mechanical and optical prob- 
lems. Still, the paradoxical aspects of quantum theory were in no 
way ameliorated, but even emphasized, by the apparent contradic- 
tion between the exigencies of the general superposition principle of 
the wave description and the feature of individuality of the ele- 
mentary atomic processes. 

* A. Einstein and P. Ehrenfest, Z. Physik, 11, 31 (1922). 

5 N. Bohr, H. A. Kramers and J. C. Slater, Phil. Mag., 41, 785 (1924). 

6 A. Einstein, Berl. Ber. 261 (1924) ; 3 and 18 (1925) . 


At the same time, Heisenberg (1925) had laid the foundation of a 
rational quantum mechanics, which was rapidly developed through 
important contributions by Born and Jordan as well as by Dirac. In 
this theory, a formalism is introduced, in which the kinematical and 
dynamical variables of classical mechanics are replaced by symbols 
subjected to a non-commutative algebra. Notwithstanding the re- 
nunciation of orbital pictures, Hamilton's canonical equations of me- 
chanics are kept unaltered and Planck's constant enters only in the 
rules of commutation 

qp - pq = 



holding for any set of conjugate variables q and p. Through a 
representation of the symbols by matrices with elements referring to 
transitions between stationary states, a quantitative formulation of 
the correspondence principle became for the first time possible. It 
may here be recalled that an important preliminary step towards 
this goal was reached through the establishment, especially by con- 
tributions of Kramers, of a quantum theory of dispersion making 
basic use of Einstein's general rules for the probability of the occur- 
rence of absorption and emission processes. 

This formalism of quantum mechanics was soon proved by Schro- 
dinger to give results identical with those obtainable by the mathe- 
matically often more convenient methods of wave theory, and in 
the following years general methods were gradually established for 
an essentially statistical description of atomic processes combining 
the features of individuality and the requirements of the superposi- 
tion principle, equally characteristic of quantum theory. Among the 
many advances in this period, it may especially be mentioned that the 
formalism proved capable of incorporating the exclusion principle 
which governs the states of systems with several electrons, and which 
already before the advent of quantum mechanics had been derived 
by Pauli from an analysis of atomic spectra. The quantitative com- 
prehension of a vast amount of empirical evidence could leave no 
doubt as to the fertility and adequacy of the quantum-mechanical 
formalism, but its abstract character gave rise to a widespread feel- 
ing of uneasiness. An elucidation of the situation should, indeed, 
demand a thorough examination of the very observational problem in 
atomic physics. 

This phase of the development was, as is well known, initiated in 
1927 by Heisenberg, 7 who pointed out that the knowledge obtainable 

*W. Heisenberg, Z. Physik, 43, 172 (1927). 



of the state of an atomic system will always involve a peculiar "in- 
determinacy." Thus, any measurement of the position of an electron 
by means of some device, like a microscope, making use of high- 
frequency radiation, will, according to the fundamental relations 
(1), be connected with a momentum exchange between the electron 
and the measuring agency, which is the greater the more accurate a 
position measurement is attempted. In comparing such considerations 
with the exigencies of the quantum-mechanical formalism, Heisen- 
berg called attention to the fact that the commutation rule (2) im- 
poses a reciprocal limitation on the fixation of two conjugate vari- 
ables, q and p, expressed by the relation 

Aq-Ap « h, 


where Aq and Ap are suitably defined latitudes in the determination 
of these variables. In pointing to the intimate connection between 
the statistical description in quantum mechanics and the actual pos- 
sibilities of measurement, this so-called indeterminacy relation is, as 
Heisenberg showed, most important for the elucidation of the para- 
doxes involved in the attempts of analyzing quantum effects with 
reference to customary physical pictures. 

The new progress in atomic physics was commented upon from 
various sides at the International Physical Congress held in Septem- 
ber 1927 at Como in commemoration of Volta. In a lecture on that 
occasion, 8 I advocated a point of view conveniently termed "com- 
plementarity," suited to embrace the characteristic features of in- 
dividuality of quantum phenomena, and at the same time to clarify 
the peculiar aspects of the observational problem in this field of ex- 
perience. For this purpose, it is decisive to recognize that, however 
far the phenomena transcend the scope of classical physical explana- 
tion, the account of all evidence must be expressed in classical terms. 
The argument is simply that by the word "experiment" we refer to 
a situation where we can tell others what we have done and what we 
have learned and that, therefore, the account of the experimental ar- 
rangement and of the results of the observations must be expressed 
in unambiguous language with suitable application of the terminology 
of classical physics. 

This crucial point, which was to become a main theme of the 
discussions reported in the following, implies the impossibility of any 
sharp separation between the behaviour of atomic objects and the 
interaction with the measuring instruments which serve to define the 

8 Atti del Congresso Internazionale dei Fisici, Como, Settembre 1927 (reprinted 
in Nature, 121, 78 and 580, 1928). 



conditions under -which the phenomena appear. In fact, the indi- 
viduality of the typical quantum effects finds its proper expression 
in the circumstance that any attempt of subdividing the phenomena 
will demand a change in the experimental arrangement introducing 
new possibilities of interaction between objects and measuring in- 
struments which in principle cannot be controlled. Consequently, 
evidence obtained under different experimental conditions cannot be 
comprehended within a single picture, but must be regarded as com- 
plementary in the sense that only the totality of the phenomena ex- 
hausts the possible information about the objects. 

Under these circumstances an essential element of ambiguity is 
involved in ascribing conventional physical attributes to atomic ob- 
jects, as is at once evident in the dilemma regarding the corpuscular 
and wave properties of electrons and photons, where we have to do 
with contrasting pictures, each referring to an essential aspect of 
empirical evidence. An illustrative example, of how the apparent 
paradoxes are removed by an examination of the experimental con- 
ditions under which the complementary phenomena appear, is also 
given by the Compton effect, the consistent description of which at 
first had presented us with such acute difficulties. Thus, any ar- 
rangement suited to study the exchange of energy and momentum 
between the electron and the photon must involve a latitude in the 
space-time description of the interaction sufficient for the definition 
of wave-number and frequency which enter into the relation (1). 
Conversely, any attempt of locating the collision between the photon 
and the electron more accurately would, on account of the unavoid- 
able interaction with the fixed scales and clocks defining the space- 
time reference frame, exclude all closer account as regards the balance 
of momentum and energy. 

As stressed in the lecture, an adequate tool for a complementary 
way of description is offered precisely by the quantum-mechanical 
formalism which represents a purely symbolic scheme permitting only 
predictions, on lines of the correspondence principle, as to results 
obtainable under conditions specified by means of classical concepts. 
It must here be remembered that even in the indeterminacy relation 
(3) we are dealing with an implication of the formalism which defies 
unambiguous expression in words suited to describe classical physical 
pictures. Thus, a sentence like "we cannot know both the momentum 
and the position of an atomic object" raises at once questions as to 
the physical reality of two such attributes of the object, which can 
be answered only by referring to the conditions for the unambiguous 
use of space-time concepts, on the one hand, and dynamical conserva- 


4 1 

tion laws, on the other hand. While the combination of these con- 
cepts into a single picture of a causal chain of events is the essence of 
classical mechanics, room for regularities beyond the grasp of such a 
description is just afforded by the circumstance that the study of the 
complementary phenomena demands mutually exclusive experimental 

The necessity, in atomic physics, of a renewed examination of the 
foundation for the unambiguous use of elementary physical ideas 
recalls in some way the situation that led Einstein to his original 
revision of the basis for all application of space-time concepts which, 
by its emphasis on the primordial importance of the observational 
problem, has lent such unity to our world picture. Notwithstanding 
all novelty of approach, causal description is upheld in relativity 
theory within any given frame of reference, but in quantum theory 
the uncontrollable interaction between the objects and the measuring 
instruments forces us to a renunciation even in such respect. This 
recognition, however, in no way points to any limitation of the scope 
of the quantum-mechanical description, and the trend of the whole 
argumentation presented in the Como lecture was to show that the 
viewpoint of complementarity may be regarded as a rational gen- 
eralization of the very ideal of causality. 

At the general discussion in Como, we all missed the presence 
of Einstein, but soon after, in October 1927, I had the opportunity 
to meet him in Brussels at the Fifth Physical Conference of the 
Solvay Institute, which was devoted to the theme "Electrons and 
Photons." At the Solvay meetings, Einstein had from their begin- 
ning been a most prominent figure, and several of us came to the 
conference with great anticipations to learn his reaction to the latest 
stage of the development which, to our view, went far in clarifying 
the problems which he had himself from the outset elicited so in- 
geniously. During the discussions, where the whole subject was re- 
viewed by contributions from many sides and where also the argu- 
ments mentioned in the preceding pages were again presented, Ein- 
stein expressed, however, a deep concern over the extent to which 
causal account in space and time was abandoned in quantum me- 

To illustrate his attitude, Einstein referred at one of the sessions 9 

9 Instkut International de Physique Solvay, Rapport et discussions du 5e Con- 
seil, Paris 1928, 253ff. 

4 2 



to the simple example, illustrated by Figure 1, of a particle (electron 
or photon) penetrating through a hole or a narrow slit in a diaphragm 
placed at some distance before a photographic plate. On account of 
the diffraction of the wave connected with the motion of the particle 
and indicated in the figure by the thin lines, it is under such condi- 
tions not possible to predict with certainty at what point the electron 
will arrive at the photographic plate, but only to calculate the proba- 
bility that, in an experiment, the electron will be found within any 
given region of the plate. The apparent difficulty, in this description, 
which Einstein felt so acutely, is the fact that, if in the experiment 
the electron is recorded at one point A of the plate, then it is out of 
the question of ever observing an effect of this electron at another 
point (B), although the laws of ordinary wave propagation offer no 
room for a correlation between two such events. 

Einstein's attitude gave rise to ardent discussions within a small 
circle, in which Ehrenfest, who through the years had been a close 
friend of us both, took part in a most active and helpful way. Surely, 
we all recognized that, in the above example, the situation presents 
no analogue to the application of statistics in dealing with complicated 
mechanical systems, but rather recalled the background for Einstein's 
own early conclusions about the unidirection of individual radiation 
effects which contrasts so strongly with a simple wave picture (cf. 
p. 36). The discussions, however, centered on the question of 
whether the quantum-mechanical description exhausted the possibili- 
ties of accounting for observable phenomena or, as Einstein main- 
tained, the analysis could be carried further and, especially, of whether 
a fuller description of the phenomena could be obtained by bringing 



into consideration the detailed balance of energy and momentum in 
individual processes. 

To explain the trend of Einstein's arguments, it may be illustrative 
here to consider some simple features of the momentum and energy 
balance in connection with the location of a particle in space and 
time. For this* purpose, we shall examine the simple case of a particle 
penetrating through a hole in a diaphragm without or with a shutter 
to open and close the hole, as indicated in Figures 2a and 2b, respec- 
tively. The equidistant parallel lines to the left in the figures indicate 
the train of plane waves corresponding to the state of motion of a 
particle which, before reaching the diaphragm, has a momentum P 
related to the wave-number a by the second of equations (1). In 
accordance with the diffraction of the waves when passing through 
the hole, the state of motion of the particle to the right of the dia- 
phragm is represented by a spherical wave train with a suitably de- 
fined angular aperture and, in case of Figure 2b, also with a limited 
radial extension. Consequently, the description of this state involves 
a certain latitude Ap in the momentum component of the particle 
parallel to the diaphragm and, in the case of a diaphragm with a 
shutter, an additional latitude AE of the kinetic energy. 

Since a measure for the latitude Aq in location of the particle in 
the plane of the diaphragm is given by the radius a of the hole, and 
since 6 <** l/aa, we get, using (1), just Ap «* 6P «=> h/Aq, in accord- 
ance with the indeterminacy relation (3). This result could, of 
course, also be obtained directly by noticing that, due to the lim- 
ited extension of the wave-field at the place of the slit, the component 
of the wave-number parallel to the plane of the diaphragm will in- 
volve a latitude Ao- « 1/a «* 1/Aq. Similarly, the spread of the fre- 







quencies of the harmonic components in the limited wave-train in 
Figure 2b is evidently Av «* \/At, where At is the time interval dur- 
ing which the shutter leaves the hole open and, thus, represents 
the latitude in time of the passage of the particle through the dia- 
phragm. From (1), we therefore get 

AE-At ~ h, (4) 

again in accordance with the relation (3) for the two conjugated 
variables E and t. 

From the point of view of the laws of conservation, the origin of 
such latitudes entering into the description of the state of the particle 
after passing through the hole may be traced to the possibilities of 
momentum and energy exchange with the diaphragm or the shutter. 
In the reference system considered in Figures 2a and 2b, the velocity 
of the diaphragm may be disregarded and only a change of momen- 
tum Ap between the particle and the diaphragm needs to be taken 
into consideration. The shutter, however, which leaves the hole 
opened during the time At, moves with a considerable velocity v «* 
a/ At, and a momentum transfer Ap involves therefore an energy ex- 
change with the particle, amounting to 

v Ap 

Aq Ap 


being just of the same order of magnitude as the latitude AE given by 
(4) and, thus, allowing for momentum and energy balance. 

The problem raised by Einstein was now to what extent a control 
of the momentum and energy transfer, involved in a location of the 
particle in space and time, can be used for a further specification of 
the state of the particle after passing through the hole. Here, it must 
be taken into consideration that the position and the motion of the 
diaphragm and the shutter have so far been assumed to be accurately 
coordinated with the space-time reference frame. This assumption 
implies, in the description of the state of these bodies, an essential 
latitude as to their momentum and energy which need not, of course, 
noticeably affect the velocities, if the diaphragm and the shutter are 
sufficiently heavy. However, as soon as we want to know the 
momentum and energy of these parts of the measuring arrangement 
with an accuracy sufficient to control the momentum and energy ex- 
change with the particle under investigation, we shall, in accordance 
with the general indeterminacy relations, lose the possibility of their 
accurate location in space and time. We have, therefore, to examine 



how far this circumstance will affect the intended use of the whole 
arrangement and, as we shall see, this crucial point clearly brings out 
the complementary character of the phenomena. 

Returning for a moment to the case of the simple arrangement 
indicated in Figure 1, it has so far not been specified to what use it 
is intended. In fact, it is only on the assumption that the diaphragm 
and the plate have well-defined positions in space that it is impossible, 
within the frame of the quantum-mechanical formalism, to make 
more detailed predictions as to the point of the photographic plate 
where the particle will be recorded. If, however, we admit a suffi- 
ciently large latitude in the knowledge of the position of the dia- 
phragm, it should, in principle, be possible to control the momentum 
transfer to the diaphragm and, thus, to make more detailed predictions 
as to the direction of the electron path from the hole to the recording 
point. As regards the quantum-mechanical description, we have to 
deal here with a two-body system consisting of the diaphragm as 
well as of the particle, and it is just with an explicit application of 
conservation laws to such a system that we are concerned in the 
Compton effect where, for instance, the observation of the recoil of 
the electron by means of a cloud chamber allows us to predict in 
what direction the scattered photon will eventually be observed. 

The importance of considerations of this kind was, in the course of 
the discussions, most interestingly illuminated by the examination of 
an arrangement where between the diaphragm with the slit and the 
photographic plate is inserted another diaphragm with two parallel 
slits, as is shown in Figure 3. If a parallel beam of electrons (or 
photons) falls from the left on the first diaphragm, we shall, under 
usual conditions, observe on the plate an interference pattern indicated 
by the shading of the photographic plate shown in front view to the 
right of the- figure. With intense beams, this pattern is built up by 







the accumulation of a large number of individual processes, each 
giving rise to a small spot on the photographic plate, and the dis- 
tribution of these spots follows a simple law derivable from the wave 
analysis. The same distribution should also be found in the statistical 
account of many experiments performed with beams so faint that in 
a single exposure only one electron (or photon) will arrive at the 
photographic plate at some spot shown in the figure as a small star. 
Since, now, as indicated by the broken arrows, the momentum trans- 
ferred to the first diaphragm ought to be different if the electron was 
assumed to pass through the upper or the lower slit in the second 
diaphragm, Einstein suggested that a control of the momentum trans- 
fer would permit a closer analysis of the phenomenon and, in par- 
ticular, make it possible to decide through which of the two slits the 
electron had passed before arriving at the plate. 

A closer examination showed, however, that the suggested control 
of the momentum transfer would involve a latitude in the knowledge 
of the position of the diaphragm which would exclude the appearance 
of the interference phenomena in question. In fact, if <o is the small 
angle between the conjectured paths of a particle passing through the 
upper or the lower slit, the difference of momentum transfer in these 
two cases will, according to (1), be equal to hoo>, and any control of 
the momentum of the diaphragm with an accuracy sufficient to meas- 
ure this difference will, due to the indeterminacy relation, involve a 
minimum latitude of the position of the diaphragm, comparable with 
1/(T«>. If, as in the figure, the diaphragm with the two slits is placed 
in the middle between the first diaphragm and the photographic plate, 
it will be seen that the number of fringes per unit length will be just 
equal to o-w and, since an uncertainty in the position of the first dia- 
phragm of the amount of 1/o-w will cause an equal uncertainty in the 
positions of the fringes, it follows that no interference effect can ap- 
pear. The same result is easily shown to hold for any other placing 
of the second diaphragm between the first diaphragm and the plate, 
and would also be obtained if, instead of the first diaphragm, another 
of these three bodies were used for the control, for the purpose sug- 
gested, of the momentum transfer. 

This point is of great logical consequence, since it is only the cir- 
cumstance that we are presented with a choice of either tracing the 
path of a particle or observing interference effects, which allows us 
to escape from the paradoxical necessity of concluding that the be- 
haviour of an electron or a photon should depend on the presence of 
a slit in the diaphragm through which it could be proved not to pass. 
We have here to do with a typical example of how the complemen- 



tary phenomena appear under mutually exclusive experimental ar- 
rangements (cf. p. 40) and are just faced with the impossibility, in 
the analysis of quantum effects, of drawing any sharp separation be- 
tween an independent behaviour of atomic objects and their inter- 
action with the measuring instruments which serve to define the con- 
ditions under which the phenomena occur. 

Our talks about the attitude to be taken in face of a novel situa- 
tion as regards analysis and synthesis of experience touched naturally 
on many aspects of philosophical thinking, but, in spite of all diver- 
gencies of approach and opinion, a most humorous spirit animated 
the discussions. On his side, Einstein mockingly asked us whether 
we could really believe that the providential authorities took recourse 
to dice-playing (". . . ob der Hebe Gott ivurfelt"), to which I replied 
by pointing at the great caution, already called for by ancient think- 
ers, in ascribing attributes to Providence in everyday language. I 
remember also how at the peak of the discussion Ehrenfest, in his 
affectionate manner of teasing his friends, jokingly hinted at the ap- 
parent similarity between Einstein's attitude and that of the opponents 
of relativity theory; but instantly Ehrenfest added that he would not 
be able to find relief in his own mind before concord with Einstein 
was reached. 

Einstein's concern and criticism provided a most valuable incentive 
for us all to reeaxamine the various aspects of the situation as regards 
the description of atomic phenomena. To me it was a welcome stimu- 
lus to clarify still further the role played by the measuring instru- 
ments and, in order to bring into strong relief the mutually exclusive 
character of the experimental conditions under which the complemen- 
tary phenomena appear, I tried in those days to sketch various appa- 
ratus in a pseudo-realistic style of which the following figures are 
examples. Thus, for the study of an interference phenomenon of 
the type indicated in Figure 3, it suggests itself to use an experimental 
arrangement like that shown in Figure 4, where the solid parts of 
the apparatus, serving as diaphragms and plate-holder, are firmly 
bolted to a common support. In such an arrangement, where the 
knowledge of the relative positions of the diaphragms and the photo- 
graphic plate is secured by a rigid connection, it is obviously im- 
possible to control the momentum exchanged between the particle 
and the separate parts of the apparatus. The only way in which, in 
such an arrangement, we could insure that the particle passed through 
one of the slits in the second diaphragm is to cover the other slit by 




a lid, as indicated in the figure; but if the slit is covered, there is of 
course no question of any interference phenomenon, and on the plate 
we shall simply observe a continuous distribution as in the case of the 
single fixed diaphragm in Figure 1. 

In the study of phenomena in the account of which we are dealing 
with detailed momentum balance, certain parts of the whole device 

figure 5 



must naturally be given the freedom to move independently of others. 
Such an apparatus is sketched in Figure 5, where a diaphragm with 
a slit is suspended by weak springs from a solid yoke bolted to the 
support on which also other immobile parts of the arrangement are to 
be fastened. The scale on the diaphragm together with the pointer 
on the bearings* of the yoke refer to such study of the motion of the 
diaphragm, as may be required for an estimate of the momentum 
transferred to it, permitting one to draw conclusions as to the deflec- 
tion suffered by the particle in passing through the slit. Since, how- 
ever, any reading of the scale, in whatever way performed, will in- 
volve an uncontrollable change in the momentum of the diaphragm, 
there will always be, in conformity with the indeterminacy prin- 
ciple, a reciprocal relationship between our knowledge of the posi- 
tion of the slit and the accuracy of the momentum control. 

In the same semi-serious style, Figure 6 represents a part of an ar- 
rangement suited for the study of phenomena which, in contrast to 
those just discussed, involve time coordination explicitly. It consists 
of a shutter rigidly connected with a robust clock resting on the 
support which carries a diaphragm and on which further parts of 
similar character, regulated by the same clock-work or by other 
clocks standardized relatively to it, are also to be fixed. The special 
aim of the figure is to underline that a clock is a piece of machinery, 
the working of which can completely be accounted for by ordinary 

figure 6 



mechanics and will be affected neither by reading of the position of 
its hands nor by the interaction between its accessories and an atomic 
particle. In securing the opening of the hole at a definite moment, 
an apparatus of this type might, for instance, be used for an accurate 
measurement of the time an electron or a photon takes to come from 
the diaphragm to some other place, but, evidently, it would leave 
no possibility of controlling the energy transfer to the shutter with 
the aim of drawing conclusions as to the energy of the particle which 
has passed through the diaphragm. If we are interested in such con- 
clusions we must, of course, use an arrangement where the shutter 
devices can no longer serve as accurate clocks, but where the knowl- 
edge of the moment when the hole in the diaphragm is open involves 
a latitude connected with the accuracy of the energy measurement 
by the general relation (4). 

The contemplation of such more or less practical arrangements 
and their more or less fictitious use proved most instructive in direct- 
ing attention to essential features of the problems. The main point 
here is the distinction between the objects under investigation and the 
measuring instruments which serve to define, in classical terms, the 
conditions under which the phenomena appear. Incidentally, we may 
remark that, for the illustration of the preceding considerations, it is 
not relevant that experiments involving an accurate control of the 
momentum or energy transfer from atomic particles to heavy bodies 
like diaphragms and shutters would be very difficult to perform, 
if practicable at all. It is only decisive that, in contrast to the proper 
measuring instruments, these bodies together with the particles would 
in such a case constitute the system to which the quantum-mechanical 
formalism has to be applied. As regards the specification of the con- 
ditions for any well-defined application of the formalism, it is more- 
over essential that the whole experimental arrangement be taken into 
account. In fact, the introduction of any further piece of apparatus, 
like a mirror, in the way of a particle might imply new interference 
effects essentially influencing the predictions as regards the results 
to be eventually recorded. 

The extent to which renunciation of the visualization of atomic 
phenomena is imposed upon us by the impossibility of their subdivision 
is strikingly illustrated by the following example to which Einstein 
very early called attention and often has reverted. If a semi-reflecting 
mirror is placed in the way of a photon, leaving two possibilities for 
its direction of propagation, the photon may either be recorded on 
one, and only one, of two photographic plates situated at great dis- 
tances in the two directions in question, or else we may, by replacing 



the plates by mirrors, observe effects exhibiting an interference be- 
tween the two reflected wave-trains. In any attempt of a pictorial 
representation of the behaviour of the photon we would, thus, meet 
with the difficulty: to be obliged to say, on the one hand, that the 
photon always chooses one of the two ways and, on the other hand, 
that it behaves as if it had passed both ways. 

It is just arguments of this kind which recall the impossibility of 
subdividing quantum phenomena and reveal the ambiguity in ascribing 
customary physical attributes to atomic objects. In particular, it must 
be realized that— besides in the account of the placing and timing of 
the instruments forming the experimental arrangement— all unambigu- 
ous use of space-time concepts in the description of atomic phenom- 
ena is confined to the recording of observations which refer to marks 
on a photographic plate or to similar practically irreversible amplifica- 
tion effects like the building of a water drop around an ion in a 
cloud-chamber. Although, of course, the existence of the quantum 
of action is ultimately responsible for the properties of the materials 
of which the measuring instruments are built and on which the func- 
tioning of the recording devices depends, this circumstance is not 
relevant for the problems of the adequacy and completeness of the 
quantum-mechanical description in its aspects here discussed. 

These problems were instructively commented upon from different 
sides at the Solvay meeting, 10 in the same session where Einstein raised 
his general objections. On that occasion an interesting discussion 
arose also about how to speak of the appearance of phenomena for 
which only predictions of statistical character can be made. The 
question was whether, as to the occurrence of individual effects, we 
should adopt a terminology proposed by Dirac, that we were con- 
cerned with a choice on the part of "nature," or, as suggested by 
Heisenberg, we should say that we have to do with a choice on the 
part of the "observer" constructing the measuring instruments and 
reading their recording. Any such terminology would, however, ap- 
pear dubious since, on the one hand, it is hardly reasonable to endow 
nature with volition in the ordinary sense, while, on the other hand, 
it is certainly not possible for the observer to influence the events 
which may appear under the conditions he has arranged. To my 
mind, there is no other alternative than to admit that, in this field of 
experience, we are dealing with individual phenomena and that our 
possibilities of handling the measuring instruments allow us only to 
make a choice between the different complementary types of phe- 
nomena we want to study. 

10 ibid., 248ff. ^_... 


The epistemological problems touched upon here were more ex- 
plicitly dealt with in my contribution to the issue of Naturwissen- 
schaften in celebration of Planck's 70th birthday in 1929. In this 
article, a comparison was also made between the lesson derived from 
the discovery of the universal quantum of action and the development 
which has followed the discovery of the finite velocity of light and 
which, through Einstein's pioneer work, has so greatly clarified basic 
principles of natural philosophy. In relativity theory, the emphasis 
on the dependence of all phenomena on the reference frame opened 
quite new ways of tracing general physical laws of unparalleled scope. 
In quantum theory, it was argued, the logical comprehension of 
hitherto unsuspected fundamental regularities governing atomic phe- 
nomena has demanded the recognition that no sharp separation can 
be made between an independent behaviour of the objects and their 
interaction with the measuring instruments which define the refer- 
ence frame. 

In this respect, quantum theory presents us with a novel situation 
in physical science, but attention was called to the very close analogy 
with the situation as regards analysis and synthesis of experience, 
which we meet in many other fields of human knowledge and inter- 
est. As is well known, many of the difficulties in psychology orig- 
inate in the different placing of the separation lines between object 
and subject in the analysis of various aspects of psychical experience. 
Actually, words like "thoughts" and "sentiments," equally indispen- 
sable to illustrate the variety and scope of conscious life, are used in 
a similar complementary way as are space-time coordination and 
dynamical conservation laws in atomic physics. A precise formula- 
tion of such analogies involves, of course, intricacies of terminology, 
and the writer's position is perhaps best indicated in a passage in the 
article, hinting at the mutually exclusive relationship which will 
always exist between the practical use of any word and attempts 
at its strict definition. The principal aim, however, of these consid- 
erations, which were not least inspired by the hope of influencing 
Einstein's attitude, was to point to perspectives of bringing general 
epistemological problems into relief by means of a lesson derived 
from the study of new, but fundamentally simple, physical experience. 

At the next meeting with Einstein at the Solvay Conference in 1930, 
our discussions took quite a dramatic turn. As an objection to the 
view that a control of the interchange of momentum and energy be- 
tween the objects and the measuring instruments was excluded if 




these instruments should serve their purpose of defining the space- 
time frame of the phenomena, Einstein brought forward the argu- 
ment that such control should be possible when the exigencies of 
relativity theory were taken into consideration. In particular, the 
general relationship between energy and mass, expressed in Einstein's 
famous formula, 

E = mc 2 , (5) 

should allow, by means of simple weighing, to measure the total 
energy of any system and, thus, in principle to control the energy 
transferred to it when it interacts with an atomic object. 

As an arrangement suited for such purpose, Einstein proposed the 
device indicated in Figure 7, consisting of a box with a hole in its 
side, which could be opened or closed by a shutter moved by means 
of a clock-work within the box. If, in the beginning, the box con- 
tained a certain amount of radiation and the clock was set to open 
the shutter for a very short interval at a chosen time, it could be 
achieved that a single photon was released through the hole at a 
moment known with as great accuracy as desired. Moreover, it 
would apparently also be possible, by weighing the whole box before 
and after this event, to measure the energy of the photon with any 
accuracy wanted, in definite contradiction to the reciprocal indeter- 
minacy of time and energy quantities in quantum mechanics. 

This argument amounted to a serious challenge and gave rise to a 
thorough examination of the whole problem. At the outcome of the 
discussion, to which Einstein himself contributed effectively, it be- 
came clear, however, that the argument could not be upheld. In 

figure 7 



fact, in the consideration of the problem, it was found necessary to 
look closer into the consequences of the identification of inertial and 
gravitational mass implied in the application of relation (5). Espe- 
cially, it was essential to take into account the relationship between 
the rate of a clock and its position in a gravitational field— well known 
from the red-shift of the lines in the sun's spectrum— following from 
Einstein's principle of equivalence between gravity effects and the 
phenomena observed in accelerated reference frames. 

Our discussion concentrated on the possible application of an appa- 
ratus incorporating Einstein's device and drawn in Figure 8 in the 
same pseudo-realistic style as some of the preceding figures. The box, 
of which a section is shown in order to exhibit its interior, is suspended 





in a spring-balance and is furnished with a pointer to read its position 
on a scale fixed to the balance support. The weighing of the box 
may thus be performed with any given accuracy Am by adjusting 
the balance to its zero position by means of suitable loads. The essen- 
tial point is now that any determination of this position with a given 
accuracy Aq w^ll involve a minimum latitude Ap in the control of the 
momentum of the box connected with Aq by the relation (3). This 
latitude must obviously again be smaller than the total impulse which, 
during the whole interval T of the balancing procedure, can be given 
by the gravitational field to a body with a mass Am, or 


— < T-g-Am, 


where g is the gravity constant. The greater the accuracy of the 
reading q of the pointer, the longer must, consequently, be the bal- 
ancing interval T, if a given accuracy Am of the weighing of the 
box with its content shall be obtained. 

Now, according to general relativity theory, a clock, when dis- 
placed in the direction of the gravitational force by an amount of 
Aq, will change its rate in such a way that its reading in the course 
of a time interval T will differ by an amount AT given by the relation 


~T c 

By comparing (6) and (7) we see, therefore, that after the weighing 
procedure there will in our knowledge of the adjustment of the 
clock be a latitude 


AT > 


~ 2 gAq. 


c 2 Am 

Together with the formula (5), this relation again leads to 

AT-AE > h, 

in accordance with the indeterminacy principle. Consequently, a 
use of the apparatus as a means of accurately measuring the energy 
of the photon will prevent us from controlling the moment of its 

The discussion, so illustrative of the power and consistency of 
relativistic arguments, thus emphasized once more the necessity of 
distinguishing, in the study of atomic phenomena, between the proper 
measuring instruments which serve to define the reference frame 


and those parts which are to be regarded as objects under investiga- 
tion and in the account of which quantum effects cannot be disre- 
garded. Notwithstanding the most suggestive confirmation of the 
soundness and wide scope of the quantum-mechanical way of descrip- 
tion, Einstein nevertheless, in a following conversation with me, ex- 
pressed a feeling of disquietude as regards the apparent lack of firmly 
laid down principles for the explanation of nature, in which all could 
agree. From my viewpoint, however, I could only answer that, in 
dealing with the task of bringing order into an entirely new field of 
experience, we could hardly trust in any accustomed principles, how- 
ever broad, apart from the demand of avoiding logical inconsistencies 
and, in this respect, the mathematical formalism of quantum mechanics 
should surely meet all requirements. 

The Solvay meeting in 1930 was the last occasion where, in com- 
mon discussions with Einstein, we could benefit from the stimulating 
and mediating influence of Ehrenfest, but shortly before his deeply 
deplored death in 1933 he told me that Einstein was far from satis- 
fied and with his usual acuteness had discerned new aspects of the 
situation which strengthened his critical attitude. In fact, by further 
examining the possibilities for the application of a balance arrange- 
ment, Einstein had perceived alternative procedures which, even if 
they did not allow the use he originally intended, might seem to 
enhance the paradoxes beyond the possibilities of logical solution. 
Thus, Einstein had pointed out that, after a preliminary weighing of 
the box with the clock and the subsequent escape of the photon, one 
was still left with the choice of either repeating the weighing or 
opening the box and comparing the reading of the clock with the 
standard time scale. Consequently, we are at this stage still free to 
choose whether we want to draw conclusions either about the energy 
of the photon or about the moment when it left the box. Without 
in any way interfering with the photon between its escape and its 
later interaction with other suitable measuring instruments, we are, 
thus, able to make accurate predictions pertaining either to the mo- 
ment of its arrival or to the amount of energy liberated by its absorp- 
tion. Since, however, according to the quantum-mechanical formal- 
ism, the specification of the state of an isolated particle cannot involve 
both a well-defined connection with the time scale and an accurate 
fixation of the energy, it might thus appear as if this formalism did 
not offer the means of an adequate description. 

Once more Einstein's searching spirit had elicited a peculiar aspect 
of the situation in quantum theory, which in a most striking manner 



illustrated how far we have here transcended customary explanation 
of natural phenomena. Still, I could not agree with the trend of his 
remarks as reported by Ehrenfest. In my opinion, there could be 
no other way to deem a logically consistent mathematical formalism 
as inadequate than by demonstrating the departure of its consequences 
from experience or by proving that its predictions did not exhaust 
the possibilities of observation, and Einstein's argumentation could be 
directed to neither of these ends. In fact, we must realize that in the 
problem in question we are not dealing with a single specified experi- 
mental arrangement, but are referring to two different, mutually ex- 
clusive arrangements. In the one, the balance together with another 
piece of apparatus like a spectrometer is used for the study of the 
energy transfer by a photon; in the other, a shutter regulated by a 
standardized clock together with another apparatus of similar kind, 
accurately timed relatively to the clock, is used for the study of the 
time of propagation of a photon over a given distance. In both these 
cases, as also assumed by Einstein, the observable effects are expected 
to be in complete conformity with the predictions of the theory. 

The problem again emphasizes the necessity of considering the 
whole experimental arrangement, the specification of which is im- 
perative for any well-defined application of the quantum-mechanical 
formalism. Incidentally, it may be added that paradoxes of the kind 
contemplated by Einstein are encountered also in such simple arrange- 
ments as sketched in Figure 5. In fact, after a preliminary measure- 
ment of the momentum of the diaphragm, we are in principle offered 
the choice, when an electron or photon has passed through the slit, 
either to repeat the momentum measurement or to control the posi- 
tion of the diaphragm and, thus, to make predictions pertaining to 
alternative subsequent observations. It may also be added that it 
obviously can make no difference, as regards observable effects ob- 
tainable by a definite experimental arrangement, whether our plans of 
constructing or handling the instruments are fixed beforehand or 
whether we prefer to postpone the completion of our planning until 
a later moment when the particle is already on its way from one in- 
strument to another. 

In the quantum-mechanical description our freedom of construct- 
ing and handling the experimental arrangement finds its proper ex- 
pression in the possibility of choosing the classically defined param- 
eters entering in any proper application of the formalism. Indeed, 
in all such respects quantum mechanics exhibits a correspondence 
with the state of affairs familiar from classical physics, which is as 


close as possible when considering the individuality inherent in the 
quantum phenomena. Just in helping to bring out this point so 
clearly, Einstein's concern had therefore again been a most welcome 
incitement to explore the essential aspects of the situation. 

The next Solvay meeting in 1933 was devoted to the problems of 
the structure and properties of atomic nuclei, in which field such 
great advances were made just in that period owing to the experi- 
mental discoveries as well as to new fruitful applications of quantum 
mechanics. It need in this connection hardly be recalled that just the 
evidence obtained by the study of artificial nuclear transformations 
gave a most direct test of Einstein's fundamental law regarding the 
equivalence of mass and energy, which was to prove an evermore im- 
portant guide for researches in nuclear physics. It may also be men- 
tioned how Einstein's intuitive recognition of the intimate relationship 
between the law of radioactive transformations and the probability 
rules governing individual radiation effects (cf. p. 35) was confirmed 
by the quantum-mechanical explanation of spontaneous nuclear dis- 
integrations. In fact, we are here dealing with a typical example of 
the statistical mode of description, and the complementary relation- 
ship between energy-momentum conservation and time-space co- 
ordination is most strikingly exhibited in the well-known paradox 
of particle penetration through potential barriers. 

Einstein himself did not attend this meeting, which took place at 
a time darkened by the tragic developments in the political world 
which were to influence his fate so deeply and add so greatly to his 
burdens in the service of humanity. A few months earlier, on a visit 
to Princeton where Einstein was then guest of the newly founded 
Institute for Advanced Study to which he soon after became per- 
manently attached, I had, however, opportunity to talk with him 
again about the epistemological aspects of atomic physics, but the 
difference between our ways of approach and expression still pre- 
sented obstacles to mutual understanding. While, so far, relatively 
few persons had taken part in the discussions reported in this article, 
Einstein's critical attitude towards the views on quantum theory 
adhered to by many physicists was soon after brought to public at- 
tention through a paper " with the title "Can Quantum-Mechanical 
Description of Physical Reality Be Considered Complete?," published 
in 1935 by Einstein, Podolsky and Rosen. 
« A. Einstein, B. Podolsky and N. Rosen, Phys. Rev., 47, 777 (1935). 


The argumentation in this paper is based on a criterion which the 
authors express in the following sentence: "If, without in any way 
disturbing a system, we can predict with certainty (i.e., with probabil- 
ity equal to unity) the value of a physical quantity, then there exists 
an element of physical reality corresponding to this physical quantity." 
By an elegant* exposition of the consequences of the quantum-me- 
chanical formalism as regards the representation of a state of a sys- 
tem, consisting of two parts which have been in interaction for a 
limited time interval, it is next shown that different quantities, the 
fixation of which cannot be combined in the representation of one 
of the partial systems, can nevertheless be predicted by measurements 
pertaining to the other partial system. According to their criterion, 
the authors therefore conclude that quantum mechanics does not 
"provide a complete description of the physical reality," and they 
express their belief that it should be possible to develop a more ade- 
quate account of the phenomena. 

Due to the lucidity and apparently incontestable character of the 
argument, the paper of Einstein, Podolsky and Rosen created a stir 
among physicists and has played a large role in general philosophical 
discussion. Certainly the issue is of a very subtle character and 
suited to emphasize how far, in quantum theory, we are beyond the 
reach of pictorial visualization. It will be seen, however, that we are 
here dealing with problems of just the same kind as those raised by 
Einstein in previous discussions, and, in an article which appeared a 
few months later, 12 I tried to show that from the point of view of 
complementarity the apparent inconsistencies were completely re- 
moved. The trend of the argumentation was in substance the same 
as that exposed in the foregoing pages, but the aim of recalling the 
way in which the situation was discussed at that time may be an 
apology for citing certain passages from my article. 

Thus, after referring to the conclusions derived by Einstein, Podol- 
sky and Rosen on the basis of their criterion, I wrote: 

Such an argumentation, however, would hardly seem suited to affect the 
soundness of quantum-mechanical description, which is based on a co- 
herent mathematical formalism covering automatically any procedure 
of measurement like that indicated. The apparent contradiction in fact 
discloses only an essential inadequacy of the customary viewpoint of 
natural philosophy for a rational account of physical phenomena of the 
type with which we are concerned in quantum mechanics. Indeed the 
finite interaction between object and measuring agencies conditioned 
by the very existence of the quantum of action entails— because of the 
impossibility of controlling the reaction of the object on the measuring 

12 N. Bohr, Phys. Rev., 48, 696 (1935). 



instruments, if these are to serve their purpose— the necessity of a final re- 
nunciation of the classical ideal of causality and a radical revision of our 
attitude towards the problem of physical reality. In fact, as we shall see, 
a criterion of reality like that proposed by the named authors contains— 
however cautious its formulation may appear— an essential ambiguity when 
it is applied to the actual problems with which we are here concerned. 

As regards the special problem treated by Einstein, Podolsky and 
Rosen, it was next shown that the consequences of the formalism 
as regards the representation of the state of a system consisting of two 
interacting atomic objects correspond to the simple arguments men- 
tioned in the preceding in connection with the discussion of the ex- 
perimental arrangements suited for the study of complementary phe- 
nomena. In fact, although any pair q and p of conjugate space and 
momentum variables obeys the rule of non-commutative multiplica- 
tion expressed by (2), and can thus only be fixed with reciprocal lati- 
tudes given by (3), the difference q x — q 2 between two space-co- 
ordinates referring to the constituents of the system will commute 
with the sum pi + p 2 of the corresponding momentum components, 
as follows directly from the commutability of q t with p 2 and q 2 
with p\. Both q x — q 2 and pi + p 2 can, therefore, be accurately 
fixed in a state of the complex system and, consequently, we can 
predict the values of either qx or pi if either q 2 or p 2 , respectively, is 
determined by direct measurements. If, for the two parts of the sys- 
tem, we take a particle and a diaphragm, like that sketched in Figure 
5, we see that the possibilities of specifying the state of the particle 
by measurements on the diaphragm just correspond to the situation 
described on p. 48 and further discussed on p. 57, where it was men- 
tioned that, after the particle has passed through the diaphragm, we 
have in principle the choice of measuring either the position of the 
diaphragm or its momentum and, in each case, making predictions as 
to subsequent observations pertaining to the particle. As repeatedly 
stressed, the principal point here is that such measurements demand 
mutually exclusive experimental arrangements. 

The argumentation of the article was summarized in the following 

From our point of view we now see that the wording of the above- 
mentioned criterion of physical reality proposed by Einstein, Podolsky 
and Rosen contains an ambiguity as regards the meaning of the expres- 
sion "without in any way disturbing a system." Of course there is in a 
case like that just considered no question of a mechanical disturbance of 
the system under investigation during the last critical stage of the measur- 
ing procedure. But even at this stage there is essentially the question of 



an influence on the very conditions which define the possible types of 
predictions regarding the future behaviour of the system. Since these 
conditions constitute an inherent element of the description of any phe- 
nomenon to which the term "physical reality" can be properly attached, 
we see that the argumentation of the mentioned authors does not justify 
their conclusion that quantum-mechanical description is essentially incom- 
plete. On the* contrary, this description, as appears from the preceding 
discussion, may be characterized as a rational utilization of all possibilities 
of unambiguous interpretation of measurements, compatible with the finite 
and uncontrollable interaction between the objects and the measuring 
instruments in the field of quantum theory. In fact, it is only the mutual 
exclusion of any two experimental procedures, permitting the unambiguous 
definition of complementary physical quantities, which provides room for 
new physical laws, the coexistence of which might at first sight appear 
irreconcilable with the basic principles of science. It is just this entirely 
new situation as regards the description of physical phenomena that the 
notion of complementarity aims at characterizing. 

Rereading these passages, I am deeply aware of the inefficiency of 
expression which must have made it very difficult to appreciate the 
trend of the argumentation aiming to bring out the essential ambiguity 
involved in a reference to physical attributes of objects when deal- 
ing with phenomena where no sharp distinction can be made between 
the behaviour of the objects themselves and their interaction with the 
measuring instruments. I hope, however, that the present account of 
the discussions with Einstein in the foregoing years, which contributed 
so greatly to make us familiar with the situation in quantum physics, 
may give a clearer impression of the necessity of a radical revision of 
basic principles for physical explanation in order to restore logical 
order in this field of experience. 

Einstein's own views at that time are presented in an article "Physics 
and Reality," published in 1936 in the Journal of the Franklin In- 
stitute. 13 Starting from a most illuminating exposition of the gradual 
development of the fundamental principles in the theories of classical 
physics and their relation to the problem of physical reality, Einstein 
here argues that the quantum-mechanical description is to be con- 
sidered merely as a means of accounting for the average behaviour 
of a large number of atomic systems, and his attitude to the belief that 
it should offer an exhaustive description of the individual phenomena 
is expressed in the following words: "To believe this is logically pos- 
sible without contradiction; but it is so very contrary to my scientific 
instinct that I cannot forego the search for a more complete con- 

13 A. Einstein, /. Franklin Inst., 221, 349 (1936). 




Even if such an attitude might seem well balanced in itself, it never- 
theless implies a rejection of the whole argumentation exposed in the 
preceding, aiming to show that, in quantum mechanics, we are not 
dealing with an arbitrary renunciation of a more detailed analysis 
of atomic phenomena, but with a recognition that such an analysis is 
in principle excluded. The peculiar individuality of the quantum 
effects presents us, as regards the comprehension of well-defined evi- 
dence, with a novel situation unforeseen in classical physics and 
irreconcilable with conventional ideas suited for our orientation and 
adjustment to ordinary experience. It is in this respect that quantum 
theory has called for a renewed revision of the foundation for the 
unambiguous use of elementary concepts as a further step in the de- 
velopment which, since the advent of relativity theory, has been so 
characteristic of modern science. 

In the following years, the more philosophical aspects of the situa- 
tion in atomic physics aroused the interest of ever larger circles and 
were, in particular, discussed at the Second International Congress for 
the Unity of Science in Copenhagen in July 1936. In a lecture on 
this occasion, 14 1 tried especially to stress the analogy in epistemologi- 
cal respects between the limitation imposed on the causal description 
in atomic physics and situations met with in other fields of knowledge. 
A principal purpose of such parallels was to call attention to the 
necessity in many domains of general human interest of facing prob- 
lems of a similar kind as those which had arisen in quantum theory 
and thereby to give a more familiar background for the apparently 
extravagant way of expression which physicists have developed to 
cope with their acute difficulties. 

Besides the complementary features conspicuous in psychology 
and already touched upon (cf. p. 52), examples of such relationships 
can also be traced in biology, especially as regards the comparison be- 
tween mechanistic and vitalistic viewpoints. Just with respect to the 
observational problem, this last question had previously been the 
subject of an address to the International Congress on Light Therapy 
held in Copenhagen in 1932, 15 where it was incidentally pointed out 
that even the psycho-physical parallelism as envisaged by Leibniz and 
Spinoza has obtained a wider scope through the development of 
atomic physics, which forces us to an attitude towards the problem 

"N. Bohr, Philosophy of Science, 4, 289 (1937). 

10 II e Congres international de la Lumiere, Copenhague 1932 (reprinted in this 
collection, p. 3). 


of explanation recalling ancient wisdom, that when searching for 
harmony in life one must never forget that in the drama of existence 
we are ourselves both actors and spectators. 

Utterances of this kind would naturally in many minds evoke the 
impression of an underlying mysticism foreign to the spirit of science; 
at the above-mentioned Congress in 1936 I therefore tried to clear 
up such misunderstandings and to explain that the only question was 
an endeavour to clarify the conditions, in each field of knowledge, 
for the analysis and synthesis of experience. 14 Yet, I am afraid that 
I had in this respect only little success in convincing my listeners, for 
whom the dissent among the physicists themselves was naturally a 
cause of scepticism about the necessity of going so far in renouncing 
customary demands as regards the explanation of natural phenomena. 
Not least through a new discussion with Einstein in Princeton in 
1937, where we did not get beyond a humourous contest concerning 
which side Spinoza would have taken if he had lived to see the de- 
velopment of our days, I was strongly reminded of the importance 
of utmost caution in all questions of terminology and dialectics. 

These aspects of the situation were especially discussed at a meeting 
in Warsaw in 1938, arranged by the International Institute of In- 
tellectual Co-operation of the League of Nations. 16 The preceding 
years had seen great progress in quantum physics owing to a num- 
ber of fundamental discoveries regarding the constitution and prop- 
erties of atomic nuclei as well as important developments of the 
mathematical formalism taking the requirements of relativity theory 
into account. In the last respect, Dirac's ingenious quantum theory 
of the electron offered a most striking illustration of the power and 
fertility of the general quantum-mechanical way of description. In 
the phenomena of creation and annihilation of electron pairs we have 
in fact to do with new fundamental features of atomicity, which are 
intimately connected with the non-classical aspects of quantum sta- 
tistics expressed in the exclusion principle, and which have demanded 
a still more far-reaching renunciation of explanation in terms of a 
pictorial representation. 

Meanwhile, the discussion of the epistemological problems in 
atomic physics attracted as much attention as ever and, in com- 
menting on Einstein's views as regards the incompleteness of the quan- 
tum-mechanical mode of description, I entered more directly on 
questions of terminology. In this connection I warned especially 
against phrases, often found in the physical literature, such as "dis- 

16 New Theories in Physics (Paris 1938), 11. 


turbing of phenomena by observation" or "creating physical attributes 
to atomic objects by measurements." Such phrases, which may serve 
to remind of the apparent paradoxes in quantum theory, are at the 
same time apt to cause confusion, since words like "phenomena" and 
"observations," just as "attributes" and "measurements," are used in 
a way hardly compatible with common language and practical defini- 

As a more appropriate way of expression I advocated the applica- 
tion of the word phenomenon exclusively to refer to the observations 
obtained under specified circumstances, including an account of the 
whole experimental arrangement. In such terminology, the observa- 
tional problem is free of any special intricacy since, in actual experi- 
ments, all observations are expressed by unambiguous statements re- 
ferring, for instance, to the registration of the point at which an elec- 
tron arrives at a photographic plate. Moreover, speaking in such a 
way is just suited to emphasize that the appropriate physical inter- 
pretation of the symbolic quantum-mechanical formalism amounts 
only to predictions, of determinate or statistical character, pertaining 
to individual phenomena appearing under conditions defined by classi- 
cal physical concepts. 

Notwithstanding all differences between the physical problems 
which have given rise to the development of relativity theory and 
quantum theory, respectively, a comparison of purely logical aspects 
of relativistic and complementary argumentation reveals striking 
similarities as regards the renunciation of the absolute significance of 
conventional physical attributes of objects. Also, the neglect of the 
atomic constitution of the measuring instruments themselves, in the 
account of actual experience, is equally characteristic of the applica- 
tions of relativity and quantum theory. Thus, the smallness of the 
quantum of action compared with the actions involved in usual ex- 
perience, including the arranging and handling of physical apparatus, 
is as essential in atomic physics as is the enormous number of atoms 
composing the world in the general theory of relativity which, as is 
often pointed out, demands that dimensions of apparatus for measur- 
ing angles can be made small compared with the radius of curvature 
of space. 

In the Warsaw lecture, I commented upon the use of not directly 
visualizable symbolism in relativity and quantum theory in the fol- 
lowing way: 

Even the formalisms, which in both theories within their scope offer ade- 
quate means of comprehending all conceivable experience, exhibit deep- 
going analogies. In fact, the astounding simplicity of the generalization of 


classical physical theories, which are obtained by the use of multidimen- 
sional geometry and non-commutative algebra, respectively, rests in both 
cases essentially on the introduction of the conventional symbol V — 1. 
The abstract character of the formalisms concerned is indeed, on closer 
examination, as typical of relativity theory as it is of quantum mechanics, 
and it is in this respect purely a matter of tradition if the former theory 
is considered as* a completion of classical physics rather than as a first 
fundamental step in the thoroughgoing revision of our conceptual means 
of comparing observations, which the modern development of physics has 
forced upon us. 

It is, of course, true that in atomic physics we are confronted with 
a number of unsolved fundamental problems, especially as regards 
the intimate relationship between the elementary unit of electric 
charge and the universal quantum of action; but these problems are 
no more connected with the epistemological points here discussed 
than is the adequacy of relativistic argumentation with the issue of 
thus far unsolved problems of cosmology. Both in relativity and in 
quantum theory we are concerned with new aspects of scientific 
analysis and synthesis and, in this connection, it is interesting to note 
that, even in the great epoch of critical philosophy in the former cen- 
tury, there was only question to what extent a priori arguments could 
be given for the adequacy of space-time coordination and causal 
connection of experience, but never question of rational generaliza- 
tions or inherent limitations of such categories of human thinking. 

Although in more recent years I have had several occasions of 
meeting Einstein, the continued discussions, from which I always 
have received new impulses, have so far not led to a common view 
about the epistemological problems in atomic physics, and our oppos- 
ing views are perhaps most clearly stated in a recent issue of Dialec- 
tica^ 1 bringing a general discussion of these problems. Realizing, 
however, the many obstacles for mutual understanding as regards a 
matter where approach and background must influence everyone's 
attitude, I have welcomed this opportunity of a broader exposition 
of the development by which, to my mind, a veritable crisis in physi- 
cal science has been overcome. The lesson we have hereby received 
would seem to have brought us a decisive step further in the never- 
ending struggle for harmony between content and form, and taught 
us once again that no content can be grasped without a formal frame 
and that any form, however useful it has hitherto proved, may be 
found to be too narrow to comprehend new experience. 

Surely, in a situation like this, where it has been difficult to reach 
mutual understanding not only between philosophers and physicists 

"N. Bohr, Dialectics 1, 312 (1948). 



but even between physicists of different schools, the difficulties have 
their root not seldom in the preference for a certain use of language 
suggesting itself from the different lines of approach. In the Insti- 
tute in Copenhagen, where through those years a number of young 
physicists from various countries came together for discussions, we 
used, when in trouble, often to comfort ourselves with jokes, among 
them the old saying of the two kinds of truth. To the one kind be- 
long statements so simple and clear that the opposite assertion obvi- 
ously could not be defended. The other kind, the so-called "deep 
truths," are statements in which the opposite also contains deep truth. 
Now, the development in a new field will usually pass through stages 
in which chaos becomes gradually replaced by order; but it is not 
least in the intermediate stage where deep truth prevails that the 
work is really exciting and inspires the imagination to search for a 
firmer hold. For such endeavours of seeking the proper balance be- 
tween seriousness and humour, Einstein's own personality stands as 
a great example and, when expressing my belief that through a 
singularly fruitful cooperation of a whole generation of physicists 
we are nearing the goal where logical order to a large extent allows 
us to avoid deep truth, I hope that it will be taken in his spirit and 
may serve as an apology for several utterances in the preceding pages. 

The discussions with Einstein which have formed the theme of 
this article have extended over many years which have witnessed 
great progress in the field of atomic physics. Whether our actual 
meetings have been of short or long duration, they have always left 
a deep and lasting impression on my mind, and when writing this 
report I have, so-to-speak, been arguing with Einstein all the time, 
even in discussing topics apparently far removed from the special 
problems under debate at our meetings. As regards the account of 
the conversations I am, of course, aware that I am relying only on 
my own memory, just as I am prepared for the possibility that many 
features of the development of quantum theory, in which Einstein 
has played so large a part, may appear to himself in a different light. 
I trust, however, that I have not failed in conveying a proper im- 
pression of how much it has meant to me to be able to benefit from 
the inspiration which we all derive from every contact with Ein- 




Before trying to answer the question to what extent we may 
speak of unity of knowledge, we may ask for the meaning of 
the word knowledge itself. It is not my intention to enter into an 
academic philosophical discourse for which I would hardly possess 
the required scholarship. Every scientist, however, is constantly 
confronted with the problem of objective description of experience, 
by which we mean unambiguous communication. Our basic tool 
is, of course, plain language which serves the needs of practical life 
and social intercourse. We shall not be concerned here with the 
origins of such language, but with its scope in scientific communica- 
tion, and especially with the problem of how objectivity may be 
retained during the growth of experience beyond the events of daily 

The main point to realize is that all knowledge presents itself 
within a conceptual framework adapted to account for previous 
experience and that any such frame may prove too narrow to com- 
prehend new experiences. Scientific research in many domains of 
knowledge has indeed time and again proved the necessity of aban- 
doning or remoulding points of view which, because of their fruit- 




fulness and apparently unrestricted applicability, were regarded as 
indispensable for rational explanation. Although such developments 
have been initiated by special studies, they entail a general lesson of 
importance for the problem of unity of knowledge. In fact, the 
widening of the conceptual framework not only has served to restore 
order within the respective branches of knowledge, but has also dis- 
closed analogies in our position with respect to analysis and synthesis 
of experience in apparently separated domains of knowledge, suggest- 
ing the possibility of an ever more embracing objective description. 

When speaking of a conceptual framework, we refer merely to the 
unambiguous logical representation of relations between experiences. 
This attitude is also apparent in the historical development in which 
formal logic is no longer sharply distinguished from studies of se- 
mantics or even philological syntax. A special role is played by 
mathematics which has contributed so decisively to the development 
of logical thinking, and which by its well-defined abstractions offers 
invaluable help in expressing harmonious relationships. Still, in our 
discussion, we shall not consider pure mathematics as a separate 
branch of knowledge, but rather as a refinement of general language, 
supplementing it with appropriate tools to represent relations for 
which ordinary verbal expression is imprecise or cumbersome. In 
this connection, it may be stressed that, just by avoiding the refer- 
ence to the conscious subject which infiltrates daily language, the use 
of mathematical symbols secures the unambiguity of definition re- 
quired for objective description. 

The development of the so-called exact sciences, characterized by 
the establishing of numerical relationships between measurements, 
has indeed been decisively furthered by abstract mathematical meth- 
ods originating from detached pursuit of generalizing logical con- 
structions. This situation is especially illustrated in physics which 
was originally understood as all knowledge concerning that nature of 
which we ourselves are part, but gradually came to mean the study of 
the elementary laws governing the properties of inanimate matter. 
The necessity, even within this comparatively simple theme, of pay- 
ing constant attention to the problem of objective description has 
deeply influenced the attitude of philosophical schools through the 
ages. In our day, the exploration of new fields of experience has 
disclosed unsuspected presuppositions for the unambiguous applica- 
tion of some of our most elementary concepts and thereby given us 
an epistemological lesson with bearings on problems far beyond the 



domain of physical science. It may therefore be convenient to start 
our discussion with a brief account of this development. 

It would carry us too far to recall in detail how, with the elimina- 
tion of mythical cosmological ideas and arguments referring to the 
purpose for our own actions, a consistent scheme of mechanics was 
built up on the basis of Galileo's pioneering work and reached such 
completion through Newton's mastery. Above all, the principles of 
Newtonian mechanics meant a far-reaching clarification of the prob- 
lem of cause and effect by permitting, from the state of a physical 
system defined at a given instant by measurable quantities, the pre- 
diction of its state at any subsequent time. It is well known how a 
deterministic or causal account of this kind led to the mechanical 
conception of nature and came to stand as an ideal of scientific ex- 
planation in all domains of knowledge, irrespective of the way knowl- 
edge is obtained. In this connection, therefore, it is important that 
the study of wider fields of physical experience has revealed the 
necessity of a closer consideration of the observational problem. 

Within its large field of application, classical mechanics presents 
an objective description in the sense that it is based on a well-defined 
use of pictures and ideas referring to the events of daily life. Still, 
however rational the idealizations used in Newtonian mechanics 
might appear, they actually went far beyond the range of experience 
to which our elementary concepts are adapted. Thus, the adequate 
use of the very notions of absolute space and time is inherently con- 
nected with the practically instantaneous propagation of light, which 
allows us to locate the bodies around us independently of their veloci- 
ties and to arrange events in a unique time sequence. However, the 
attempt to develop a consistent account of electromagnetic and optical 
phenomena revealed that observers moving relative to each other 
with large velocities will coordinate events differently. Not only 
may such observers take a different view of shapes and positions of 
rigid bodies, but events at separate points of space which to one ob- 
server appear as simultaneous may be judged by another as occur- 
ring at different times. 

Far from giving rise to confusion and complication, the explora- 
tion of the extent to which the account of physical phenomena de- 
pends on the standpoint of the observer proved an invaluable guide 
in tracing general physical laws common to all observers. Retaining 
the idea of determinism, but relying only on relations between un- 



ambiguous measurements referring ultimately to coincidences of 
events, Einstein succeeded in remoulding and generalizing the whole 
edifice of classical physics and in lending to our world picture a unity 
surpassing all previous expectations. In the general theory of rela- 
tivity, the description is based on a curved four-dimensional space- 
time metric which automatically accounts for gravitational effects and 
the singular role of the speed of light signals representing an upper 
limit for any consistent use of the physical concept of velocity. The 
introduction of such unfamiliar but well-defined mathematical ab- 
stractions in no way implies ambiguity but rather offers an instructive 
illustration of how a widening of the conceptual framework affords 
the appropriate means of eliminating subjective elements and enlarg- 
ing the scope of objective description. 

New, unsuspected aspects of the observational problem were dis- 
closed by the exploration of the atomic constitution of matter. As is 
well known, the idea of a limited divisibility of substances, introduced 
to explain the persistence of their characteristic properties in spite 
of the variety of natural phenomena, goes back to antiquity. Still, 
almost to our day, such views were regarded as essentially hypo- 
thetical in the sense that they seemed inaccessible to direct con- 
firmation by observation because of the coarseness of our sense or- 
gans and tools, themselves composed of innumerable atoms. Never- 
theless, with the great progress in chemistry and physics in the last 
centuries, atomic ideas proved increasingly fruitful. In particular, 
the direct application of classical mechanics to the interaction of 
atoms and molecules during their incessant motions led to a general 
understanding of the principles of thermodynamics. 

In this century, the study of newly discovered properties of mat- 
ter such as natural radioactivity has convincingly confirmed the 
foundations of atomic theory. In particular, through the develop- 
ment of amplification devices, it has been possible to study phenomena 
essentially dependent on single atoms, and even to obtain extensive 
knowledge of the structure of atomic systems. The first step was 
the recognition of the electron as a common constituent of all sub- 
stances, and an essential completion of our ideas of atomic constitu- 
tion was obtained by Rutherford's discovery of the atomic nucleus 
which contains within an extremely small volume almost the whole 
mass of the atom. The invariability of the properties of the elements 
in ordinary physical and chemical processes is directly explained by 
the circumstance that in such processes, although the electron bind- 
ing may be largely influenced, the nucleus remains unaltered. With 
his demonstration of the transmutability of atomic nuclei by more 


7 1 

powerful agencies, Rutherford, however, opened a quite new field of 
research, often referred to as modern alchemy, which, as is well 
known, was eventually to lead to the possibility of releasing immense 
amounts of energy stored in atomic nuclei. 

Although many fundamental properties of matter were explained 
by the simple pitture of the atom, it was evident from the beginning 
that classical ideas of mechanics and electromagnetism did not suffice 
to account for the essential stability of atomic structures, as ex- 
hibited by the specific properties of the elements. However, a clue 
to the elucidation of this problem was afforded by the discovery of 
the universal quantum of action to which Planck was led in the first 
year of our century by his penetrating analysis of the laws of thermal 
radiation. This discovery revealed in atomic processes a feature of 
wholeness quite foreign to the mechanical conception of nature, and 
made it evident that the classical physical theories are idealizations 
valid only in the description of phenomena in the analysis of which 
all actions are sufficiently large to permit the neglect of the quan- 
tum. While this condition is amply fulfilled in phenomena on the 
ordinary scale, we meet in atomic phenomena regularities of quite a 
new kind, defying deterministic pictorial description. 

A rational generalization of classical physics, allowing for the ex- 
istence of the quantum but retaining the unambiguous interpretation 
of the experimental evidence defining the inertial mass and electric 
charge of the electron and the nucleus, presented a very difficult task. 
By concerted efforts of a whole generation of theoretical physicists, 
a consistent and, within a wide scope, exhaustive description of atomic 
phenomena was, however, gradually developed. This description 
makes use of a mathematical formalism in which the variables in the 
classical physical theories are replaced by symbols subject to a non- 
commutable algorism involving Planck's constant. Owing to the very 
character of such "mathematical abstractions, the formalism does not 
allow pictorial interpretation on accustomed lines, but aims directly 
at establishing relations between observations obtained under well- 
defined conditions. Corresponding to the circumstance that differ- 
ent individual quantum processes may take place in a given experi- 
mental arrangement, these relations are of an inherently statistic 

By means of the quantum mechanical formalism, a detailed account 
of an immense amount of experimental evidence regarding the physi- 
cal and chemical properties of matter has been achieved. Moreover, 
by adapting the formalism to the exigencies of relativistic invariance, 
it has been possible, within wide limits, to order the rapidly growing 






new knowledge concerning the properties of elementary particles 
and the constitution of atomic nuclei. Notwithstanding the astound- 
ing power of quantum mechanics, the radical departure from accus- 
tomed physical explanation, and especially the renunciation of the 
very idea of determinism, has given rise to doubts in the minds of 
many physicists and philosophers as to whether we are here dealing 
with, a temporary expedient or are confronted with an irrevocable 
step as regards objective description. The clarification of this prob- 
lem has actually demanded a radical revision of the fundamentals to 
the description and comprehension of physical experience. 

In this context, we must recognize above all that, even when the 
phenomena transcend the scope of classical physical theories, the 
account of the experimental arrangement and the recording of ob- 
servations must be given in plain language, suitably supplemented 
by technical physical terminology. This is a clear logical demand, 
since the very word "experiment" refers to a situation where we can 
tell others what we have done and what we have learned. However, 
the fundamental difference with respect to the analysis of phenomena 
in classical and in quantum physics is that in the former the inter- 
action between the objects and the measuring instruments may be 
neglected or compensated for, while in the latter this interaction 
forms an integral part of the phenomena. The essential wholeness 
of a proper quantum phenomenon finds indeed logical expression in 
the circumstance that any attempt at its well-defined subdivision 
would require a change in the experimental arrangement incompatible 
with the appearance of the phenomenon itself. 

In particular, the impossibility of a separate control of the inter- 
action between the atomic objects and the instruments indispensable 
for the definition of the experimental conditions prevents the unre- 
stricted combination of space-time coordination and dynamical con- 
servation laws on which the deterministic description in classical 
physics rests. In fact, any unambiguous use of the concepts of space 
and time refers to an experimental arrangement involving a transfer 
of momentum and energy, uncontrollable in principle, to fixed scales 
and synchronized clocks which are required for the definition of the 
reference frame. Conversely, the account of phenomena which are 
characterized by the laws of conservation of momentum and energy 
involves in principle a renunciation of detailed space-time coordina- 
tion. These circumstances find quantitative expression in Heisen- 
berg's indeterminacy relations which specify the reciprocal latitude 
for the fixation of kinematical and dynamical variables in the defini- 
tion of the state of a physical system. In accordance with the char- 

acter of the quantum mechanical formalism, such relations cannot, 
however, be interpreted in terms of attributes of objects referring to 
classical pictures, but we are here dealing with the mutually exclu- 
sive conditions for the unambiguous use of the very concepts of space 
and time on the one hand, and of dynamical conservation laws on 
the other. v 

In this context, one sometimes speaks of "disturbance of phe- 
nomena by observation" or "creation of physical attributes to atomic 
objects by measurements." Such phrases, however, are apt to cause 
confusion, since words like phenomena and observation, just as at- 
tributes and measurements, are here used in a way incompatible with 
common language and practical definition. On the lines of objective 
description, it is indeed more appropriate to use the word phenome- 
non to refer only to observations obtained under circumstances whose 
description includes an account of the whole experimental arrange- 
ment. In such terminology, the observational problem in quantum 
physics is deprived of any special intricacy and we are, moreover, 
directly reminded that every atomic phenomenon is closed in the 
sense that its observation is based on registrations obtained by means 
of suitable amplification devices with irreversible functioning such 
as, for example, permanent marks on a photographic plate, caused 
by the penetration of electrons into the emulsion. In this connection, 
it is important to realize that the quantum-mechanical formalism 
permits well-defined applications referring only to such closed phe- 
nomena. Also in this respect it represents a rational generalization 
of classical physics in which every stage of the course of events is 
described by measurable quantities. 

The freedom of experimentation, presupposed in classical physics, 
is of course retained and corresponds to the free choice of experimen- 
tal arrangements for which the mathematical structure of the quantum 
mechanical formalism offers the appropriate latitude. The circum- 
stance that, in general, one and the same experimental arrangement 
may yield different recordings is sometimes picturesquely described 
as a "choice of nature" between such possibilities. Needless to say, 
such a phrase implies no allusion to a personification of nature, but 
simply points to the impossibility of ascertaining on accustomed lines 
directives for the course of a closed indivisible phenomenon. Here, 
logical approach cannot go beyond the deduction of the relative 
probabilities for the appearance of the individual phenomena under 
given experimental conditions. In this respect, quantum mechanics 
presents a consistent generalization of deterministic mechanical de- 
scription which it embraces as an asymptotic limit in the case of 



physical phenomena on a scale sufficiently large to allow the neglect 
of the quantum of action. 

A most conspicuous characteristic of atomic physics is the novel 
relationship between phenomena observed under experimental con- 
ditions demanding different elementary concepts for their descrip- 
tion. Indeed, however contrasting such experiences might appear 
when attempting to picture a course of atomic processes on classical 
lines, they have to be considered as complementary in the sense 
that they represent equally essential knowledge about atomic systems 
and together exhaust this knowledge. The notion of complementarity 
does in no way involve a departure from our position as detached 
observers of nature, but must be regarded as the logical expression 
of our situation as regards objective description in this field of ex- 
perience. The recognition that the interaction between the measur- 
ing tools and the physical systems under investigation constitutes an 
integral part of quantum phenomena has not only revealed an un- 
suspected limitation of the mechanical conception of nature, as 
characterized by attribution of separate properties to physical sys- 
tems, but has forced us, in the ordering of experience, to pay proper 
attention to the conditions of observation. 

Returning to the much debated question of what has to be de- 
manded of a physical explanation, one must keep in mind that classi- 
cal mechanics had already implied the renunciation of a cause for 
uniform motion and furthermore that relativity theory has taught us 
how arguments of invariance and equivalence must be treated as 
categories of rational explanation. Similarly, in the complementary 
description of quantum physics, we have to do with a further self- 
consistent generalization which permits the inclusion of regularities 
decisive for the account of fundamental properties of matter, but 
which transcends the scope of deterministic description. The history 
of physical science thus demonstrates how the exploration of ever 
wider fields of experience, in revealing unsuspected limitations of 
accustomed ideas, indicates new ways of restoring logical order. As 
we shall now proceed to show, the epistemological lesson contained 
in the development of atomic physics reminds us of similar situa- 
tions with respect to the description and comprehension of experi- 
ence far beyond the borders of physical science, and allows us to 
trace common features promoting the search for unity of knowledge. 

The first problem with which we are confronted when leaving the 
proper domain of physics is the question of the place of living or- 
ganisms in the description of natural phenomena. Originally, no 



sharp distinction between animate and inanimate matter was made, 
and it is well known that Aristotle, in stressing the wholeness of the 
individual organisms, opposed the views of the atomists, and even in 
the discussion of the foundations of mechanics retained ideas like 
purpose and potency. However, as a result of the great discoveries 
in anatomy arid physiology at the time of the Renaissance, and espe- 
cially of the advent of classical mechanics in the deterministic de- 
scription of which any reference to purpose is eliminated, a com- 
pletely mechanistic conception of nature suggested itself, and a large 
number of organic functions could in fact be accounted for by the 
same physical and chemical properties of matter which found far- 
reaching explanation on simple atomic ideas. It is true that the 
structure and functioning of organisms involve an ordering of 
atomic processes which has sometimes seemed difficult to reconcile 
with the laws of thermodynamics, implying a steady approach to- 
wards disorder among the atoms constituting an isolated physical 
system. If, however, sufficient account is taken of the circumstance 
that the free energy necessary to maintain and develop organic sys- 
tems is continually supplied from their surroundings by nutrition 
and respiration, it becomes clear that there is in such respect no 
question of any violation of general physical laws. 

In the last decades, great advances have been achieved in our 
knowledge of the structure and functioning of organisms, and in 
particular it has become evident that quantum regularities in many 
respects here play a fundamental role. Not only are such regulari- 
ties basic of the remarkable stability of the highly complex molecular 
structures which form the essential constituents of the cells respon- 
sible for the hereditary properties of the species, but research on 
mutations produced by exposing organisms to penetrating radiation 
offers a striking application of the statistical laws of quantum physics. 
Also, the sensitivity of perceptive organs, so important for the in- 
tegrity of the organisms, has been found to approach the level of 
individual quantum processes, and amplification mechanisms play an 
important part especially in the transmission of nervous messages. 
The whole development has again, although in a novel manner, 
brought the mechanistic approach to biological problems to the fore- 
ground, but at the same time the question has become acute as to 
whether a comparison between the organisms and highly complex 
and refined systems, such as modern industrial constructions or elec- 
tronic calculation machines, offers the proper basis for an objective 
description of the self-regulating entities which living organisms 

7 6 


Returning to the general epistemological lesson which atomic 
physics has given us, we must in the first place realize that the closed 
processes studied in quantum physics are not directly analogous to 
biological functions for the maintenance of which a continual ex- 
change of matter and energy between the organism and the environ- 
ments is required. Moreover, any experimental arrangement which 
would permit control of such functions to the extent demanded for 
their well-defined description in physical terms would be prohibitive 
to the free display of life. This very circumstance, however, sug- 
gests an attitude to the problem of organic life providing a more ap- 
propriate balance between a mechanistic and a finalistic approach. 
In fact, just as the quantum of action appears in the account of atomic 
phenomena as an element for which an explanation is neither possible 
nor required, the notion of life is elementary in biological science 
where, in the existence and evolution of living organisms, we are con- 
cerned with manifestations of possibilities in that nature to which we 
belong rather than with the outcome of experiments which we can 
ourselves perform. Actually, we must recognize that the require- 
ments of objective description, in tendency at least, are fulfilled by 
the characteristic complementary way in which arguments based on 
the full resources of physical and chemical science, and concepts 
directly referring to the integrity of the organism transcending the 
scope of these sciences, are practically used in biological research. 
The main point is that only by renouncing an explanation of life in 
the ordinary sense do we gain a possibility of taking into account 
its characteristics. 

Of course, in biology just as in physics, we retain our position as 
detached observers, and the question is only that of the different 
conditions for the logical comprehension of experience. This ap- 
plies also to the study of the innate and conditioned behaviour of ani- 
mals and man to which psychological concepts readily lend them- 
selves. Even in an allegedly behaviouristic approach, it is hardly 
possible to avoid such concepts, and the very idea of consciousness 
presents itself when we deal with behaviour of so high a degree of 
complexity that its description virtually involves introspection on 
the part of the individual organism. We have here to do with 
mutually exclusive applications of the words instinct and reason, illus- 
trated by the degree to which instinctive behaviour is suppressed in 
human societies. Although we meet in trying to account for the 
state of our mind ever greater difficulties as regards observational 
detachment, it is still possible to uphold the requirements of objective 
description to a great extent even in human psychology. In this 



connection, it is interesting to note that, while in the early stages of 
physical science one could directly rely on such features of the events 
of daily life which permitted a simple causal account, an essentially 
complementary description of the content of our mind has been used 
since the origin of languages. In fact, the rich terminology adapted 
to such communication does not point to an unbroken course of 
events, but rather to mutually exclusive experiences characterized by 
different separations between the content on which attention is fo- 
cused and the background indicated by the word ourselves. 

An especially striking example is offered by the relationship be- 
tween situations in which we ponder on the motives for our actions 
and in which we experience a feeling of volition. In normal life, 
such shifting of the separation is more or less intuitively recognized, 
but symptoms characterized as "confusion of the egos," which may 
lead to dissolution of the personality, are well known in psychiatry. 
The use of apparently contrasting attributes referring to equally 
important aspects of the human mind presents indeed a remarkable 
analogy to the situation in atomic physics, where complementary 
phenomena for their definition demand different elementary con- 
cepts. Above all, the circumstance that the very word "conscious" 
refers to experiences capable of being retained in the memory sug- 
gests a comparison between conscious experiences and physical ob- 
servations. In such an analogy, the impossibility of providing an 
unambiguous content to the idea of subconsciousness corresponds to 
the impossibility of pictorial interpretation of the quantum-mechani- 
cal formalism. Incidentally, psychoanalytical treatment of neuroses 
may be said to restore balance in the content of the memory of the 
patient by bringing him new conscious experience, rather than by 
helping him to fathom the abysses of his subconsciousness. 

From a biological point of view, we can only interpret the char- 
acteristics of psychical phenomena by concluding that every con- 
scious experience corresponds to a residual impression in the organ- 
ism, amounting to an irreversible recording in the nervous system 
of the outcome of processes which are not open to introspection and 
hardly adapted to exhaustive definition by mechanistic approach. 
Certainly, such recordings in which the interplay of numerous nerve 
cells is involved are essentially different from the permanent struc- 
tures in any single cells of the organism which are connected with 
genetic reproduction. From a finalistic point of view, however, we 
may stress not only the usefulness of permanent recordings in their 
influence on our reactions to subsequent stimuli, but equally the im- 
portance that later generations are not encumbered by the actual 



experiences of individuals but rely only on the reproduction of such 
properties of the organism as have proved serviceable for the collec- 
tion and utilization of knowledge. In any attempt to pursue the en- 
quiry we must, of course, be prepared to meet increasing difficulties 
at every step, and it is suggestive that the simple concepts of physical 
science lose their immediate applicability to an ever higher degree 
the more we approach the features of living organisms related to the 
characteristics of our mind. 

To illustrate the argument, we may briefly refer to the old problem 
of free will. From what has already been said it is evident that the 
word volition is indispensable to an exhaustive description of psychi- 
cal phenomena, but the problem is how far we can speak about free- 
dom to act according to our possibilities. As long as unrestricted 
deterministic views are taken, the idea of such freedom is of course 
excluded. However, the general lesson of atomic physics, and in 
particular of the limited scope of mechanistic description of bio- 
logical phenomena, suggests that the ability of organisms to adjust 
themselves to environment includes the power of selecting the most 
appropriate way to this purpose. Because it is impossible to judge 
such questions on a purely physical basis, it is most important to 
recognize that psychological experience may offer more pertinent 
information on the problems. The decisive point is that, if we 
attempt to predict what another person will decide to do in a 
given situation, not only must we strive to know his whole back- 
ground, including the story of his life in all respects which may 
have contributed to form his character, but we must realize that what 
we are ultimately aiming at is to put ourselves in his place. Of 
course, it is impossible to say whether a person wants to do some- 
thing because he believes he can, or whether he can because he will, 
but it is hardly disputable that we have the feeling of, so-to-speak, 
being able to make the best out of the circumstance. From the point 
of view of objective description, nothing can here be added or 
taken away, and in this sense we may both practically and logically 
speak of freedom of will in a way which leaves the proper latitude for 
the use of words like responsibility and hope, which themselves 
are as little definable separately as other words indispensable to human, 

Such considerations point to the epistemological implications of 
the lesson regarding our observational position, which the develop- 
ment of physical science has impressed upon us. In return for the 
renunciation of accustomed demands on explanation, it offers a logi- 
cal means of comprehending wider fields of experience, necessitat- 



ing proper attention to the placing of the object-subject separation. 
Since, in philosophical literature, reference is sometimes made to 
different levels of objectivity or subjectivity or even of reality, it 
may be stressed that the notion of an ultimate subject as well as 
conceptions like realism and idealism find no place in objective de- 
scription as we* have defined it; but this circumstance of course does 
not imply any limitation of the scope of the enquiry with which we 
are concerned. 

Having touched upon some of the problems in science which 
relate to the unity of knowledge, I shall turn to the further question 
raised in our programme, whether there is a poetical or spiritual or 
cultural truth distinct from scientific truth. With all the reluctance 
of a scientist to enter into such fields, I shall venture, with an atti- 
tude similar to that indicated in the preceding, to comment on this 
question. Taking up the argument of the relation between our 
means of expression and the field of experience with which we are 
concerned, we are indeed directly confronted with the relationship 
of science and art. The enrichment which art can give us originates 
in its power to remind us of harmonies beyond the grasp of systematic 
analysis. Literary, pictorial and musical art may be said to form a 
sequence of modes of expression, where the ever more extensive 
renunciation of definition, characteristic of scientific communication, 
leaves fantasy a freer display. In particular, in poetry this purpose 
is achieved by the juxtaposition of words related to shifting observa- 
tional situations, thereby emotionally uniting manifold aspects of hu- 
man knowledge. 

Notwithstanding the inspiration required in all work of art, it may 
not be irreverent to remark that even at the climax of his work the 
artist relies on the common human foundation on which we stand. 
In particular, we must realize that a word like improvisation, which 
comes so readily to the tongue when speaking of artistic achievements, 
points to a feature essential to all communication. Not only are we 
in ordinary conversation more or less unaware of the verbal expres- 
sions we are going to choose in communicating what is on our minds, 
but even in written papers, where we have the possibility of recon- 
sidering every word, the question whether to let it stand or change 
it demands for its answer a final decision essentially equivalent to an 
improvisation. Incidentally, in the balance between seriousness and 
humour, characteristic of all truly artistic achievements, we are re- 
minded of complementary aspects conspicuous in children's play and 



no less appreciated in mature life. Indeed, if we always endeavour 
to speak quite seriously, we run the risk of very soon appearing ridicu- 
lously tedious to our listeners and ourselves, but if we try to joke all 
the time, we soon find ourselves, and our listeners too, in the desperate 
mood of the jesters in Shakespeare's dramas. 

In a comparison between science and art, we must of course not 
forget that in the former we have to do with systematic concerted 
efforts to augment experience and develop appropriate concepts for 
its comprehension, resembling the carrying and fitting of stones to a 
building, while in the latter we are presented with more intuitive 
individual endeavours to evoke sentiments which recall the whole- 
ness of our situation. We are here at a point where the question of 
unity of knowledge evidently contains ambiguity, like the word 
"truth" itself. Indeed, with respect to spiritual and cultural values we 
are also reminded of epistemological problems related to the proper 
balance between our desire for an all-embracing way of looking at life 
in its multifarious aspects and our power of expressing ourselves in a 
logically consistent manner. 

Here, essentially different starting points are taken by science, 
aiming at the development of general methods for ordering common 
human experience, and religions, originating in endeavours to fur- 
ther harmony of outlook and behaviour within communities. Of 
course, in any religion, all knowledge shared by the members of the 
community was included in the general framework, a primary con- 
tent of which were the values and ideals emphasized in cult and 
faith. Therefore, the inherent relation between content and frame 
hardly demanded attention until the subsequent progress of science 
entailed a novel cosmological or epistemological lesson. The course 
of history presents many illustrations in such respects, and we may 
refer especially to the veritable schism between science and reli- 
gion which accompanied the development of the mechanical con- 
ception of nature at the time of the European Renaissance. On the 
one hand, many phenomena, hitherto regarded as manifestations of 
divine providence, appeared as consequences of general immutable 
laws of nature. On the other hand, the physical methods and view- 
points were far remote from the emphasis on human values and 
ideals essential to religion. Common to the schools of so-called 
empirical and critical philosophy, there prevailed therefore an atti- 
tude of more or less vague distinction between objective knowledge 
and subjective belief. 

In emphasizing the necessity in unambiguous communication of 
paying proper attention to the placing of the object-subject separa- 



tion, modern development of science has, however, created a new 
basis for the use of such words as knowledge and belief. Above all, 
the recognition of inherent limitations in the notion of causality has 
offered a frame in which the idea of universal predestination is re- 
placed by the concept of natural evolution. With respect to the 
organization of human societies, we may particularly stress that de- 
scription of the position of the individual within his community 
presents typically complementary aspects related to the shifting bor- 
der between the appreciation of values and the background on which 
they are judged. Surely, every stable human society demands fair 
play specified in judicial rules, but at the same time, life without 
attachment to family and friends would obviously be deprived of 
some of its most precious values. Still, though the closest possible 
combination of justice and charity presents a common goal in all 
cultures, it must be recognized that any occasion which calls for the 
strict application of law has no room for the display of charity and 
that, conversely, benevolence and compassion may conflict with all 
ideas of justice. This point, in many religions mythically illustrated 
by the fight between deities personifying such ideals, is stressed in 
old Oriental philosophy in the admonition never to forget as we 
search for harmony in human life that on the scene of existence we 
are ourselves actors as well as spectators. 

In comparing different cultures resting on traditions fostered by 
historical events, we meet with the difficulty of appreciating the cul- 
ture of one nation on the background of traditions of another. In 
this respect, the relation between national cultures has sometimes 
been described as complementary, although this word cannot here 
be taken in the strict sense in which it is used in atomic physics or in 
psychological analysis, where we are dealing with invariable char- 
acteristics of -our situation. In fact, not only has contact between 
nations often resulted in the fusion of cultures retaining valuable 
elements of national traditions, but anthropological research is stead- 
ily becoming a most important source for illuminating common fea- 
tures of cultural developments. Indeed, the problem of unity of 
knowledge can hardly be separated from the striving for universal 
understanding as a means of elevating human culture. 

In concluding this address, I feel that I ought to apologize for 
speaking on such general topics with so much reference to the special 
field of knowledge represented by physical science. I have tried, 
however, to indicate a general attitude suggested by the serious les- 



son we have in our day received in this field and which to me appears 
of importance for the problem of unity of knowledge. This attitude 
may be summarized by the endeavour to achieve a harmonious com- 
prehension of ever wider aspects of our situation, recognizing that 
no experience is definable without a logical frame and that any ap- 
parent disharmony can be removed only by an appropriate widening 
of the conceptual framework. 



Human Knowledge 


In the history of science, this century's exploration of the world of 
atoms has hardly any parallel in so far as the progress of knowl- 
edge and the mastery of that nature of which we ourselves are part 
are concerned. However, with every increase of knowledge and 
abilities is connected a greater responsibility; and the fulfilment of 
the rich promise and the elimination of the new dangers of the atomic 
age confront our whole civilization with a serious challenge which 
can be met only by cooperation of all peoples, resting on a mutual 
understanding of the human fellowship. In this situation, it is im- 
portant to realize that science, which knows no national boundaries 
and whose achievements are the common possession of mankind, 
has through the ages united men in their efforts to elucidate the 
foundations of our knowledge. As I shall attempt to show, the study 
of atoms, which was to entail such far-reaching consequences and 
whose progress has been based on world-wide cooperation, not only 
has deepened our insight into a new domain of experience, but has 
thrown new light on general problems of knowledge. 

At first, it might seem surprising that atomic science should con- 
tain a lesson of a general nature, but we must remember that it has 



in all stages of its development concerned profound problems of 
knowledge. Thus, thinkers of antiquity, by assuming a limit for the 
divisibility of substances, attempted to find a basis for understanding 
the features of permanency exhibited by natural phenomena, in spite 
of their multifariousness and variability. Although atomic ideas have 
contributed more and more fruitfully to the development of physics 
and chemistry since the Renaissance, they were regarded as a hy- 
pothesis right up to the beginning of this century. Indeed, it was 
taken for granted that our sense organs, themselves composed of in- 
numerable atoms, were too coarse to observe the smallest parts of 
matter. This situation was, however, to become essentially changed 
by the great discoveries at the turn of the century and, as is well 
known, progress in experimental technique made it possible to record 
the effects of single atoms and to obtain information on the more 
elementary particles of which the atoms themselves were found to 
be composed. 

In spite of the deep influence exerted by ancient atomism on the 
development of the mechanical conception of nature, it was the 
study of immediately accessible astronomical and physical experi- 
ence which made it possible to trace the regularities expressed in 
the so-called classical physics. Galileo's dictum, according to which 
the account of phenomena should be based on measurable quantities, 
made it possible to eliminate such animistic views which had so long 
hindered the rational formulation of mechanics. In Newton's prin- 
ciples, the foundation was laid of a deterministic description per- 
mitting, from the knowledge of the state of a physical system at a 
given moment, prediction of its state at any subsequent time. On 
the same lines, it was possible to account for electromagnetic phe- 
nomena. This required, however, that the description of the state of 
the system should include, besides the positions and velocities of the 
electrified and magnetized bodies, the strength and direction of the 
electrical and magnetic forces at every point of space, at the given 

The conceptual framework which is characteristic of classical 
physics was long thought to provide the correct tool for the de- 
scription of all physical phenomena, and not least was it suited to the 
utilization and development of atomic ideas. Of course, for systems, 
such as ordinary bodies which are composed of an enormous number 
of constituent parts, there could be no question of an exhaustive 
description of the state of the system. Without abandoning the 
deterministic ideal, it became possible, however, on the basis of the 
principles of classical mechanics, to deduce statistical regularities re- 


fleeting many of the properties of material bodies. Even though 
the mechanical laws of motion permit a complete reversal of the 
course of single processes, full explanation of the characteristic fea- 
ture of irreversibility in heat phenomena was found in the statistical 
energy equilibrium resulting from the interaction of the molecules. 
This great extension of the application of mechanics emphasized 
further the indispensability of atomic ideas to the description of 
nature and opened the first possibilities of counting the atoms of 
the substances. 

However, clarification of the foundation of the laws of thermo- 
dynamics was to open the way for recognition of a feature of whole- 
ness in atomic processes far beyond the old doctrine of the limited 
divisibility of matter. As is well known, the closer analysis of heat 
radiation became the test of the scope of classical physical ideas. 
The discovery of electromagnetic waves had already provided a 
basis for understanding the propagation of light, explaining many of 
the optical properties of substances; but endeavours to account for 
radiation equilibrium confronted such ideas with insurmountable 
difficulties. The circumstance that one had here to do with argu- 
ments based on general principles and quite independent of special 
assumptions regarding the constituents of the substances led Planck, 
in the first year of this century, to the discovery of the universal 
quantum of action, which showed clearly that the classical physical 
description is an idealization of limited applicability. In phenomena 
on the ordinary scale, the actions involved are so large compared to 
the quantum that it can be left out of consideration. However, in 
proper quantum processes, we meet regularities which are completely 
foreign to the mechanical conception of nature and which defy pic- 
torial deterministic description. 

The task with which Planck's discovery confronted physicists was 
nothing less than, by means of a thorough analysis of the presuppo- 
sitions on which the application of our most elementary concepts 
are based, to provide room for the quantum of action in a rational 
generalization of the classical physical description. During the de- 
velopment of quantum physics, entailing so many surprises, we have 
time and again been reminded of the difficulties of orienting ourselves 
in a domain of experience far from that to the description of which 
our means of expression are adapted. Rapid progress has been made 
possible by a wide and intensive collaboration among physicists from 
many countries, whose diverse approaches have helped in a most 



fruitful way to focus the problem ever more sharply. On this occa- 
sion, of course, it will not be possible to deal in detail with individual 
contributions, but as a background for the following considerations 
I shall remind you briefly of some of the main features of the, 

While Planck cautiously limited himself to statistical arguments 
and emphasized the difficulties of abandoning the classical founda- 
tions in the detailed description of nature, Einstein daringly pointed 
to the necessity of taking the quantum of action into account in in- 
dividual atomic phenomena. In the same year that he so har- 
moniously completed the framework of classical physics by estab- 
lishing the theory of relativity, he showed that the description of 
observations on photoelectric effects requires that the transmission 
of energy to each of the electrons expelled from the substances 
corresponds to the absorption of a so-called quantum of radiation. 
Since the idea of waves is indispensable to the account of the propa- 
gation of light, there could be no question of simply replacing it with 
a corpuscular description, and one was therefore confronted with a 
peculiar dilemma whose solution was to require a thorough analysis 
of the scope of pictorial concepts. 

As is well known, this question was further accentuated by Ruth- 
erford's discovery of the atomic nucleus which, despite its minute- 
ness, contains almost the whole mass of the atom and whose electrical 
charge corresponds to the number of electrons in the neutral atom. 
This gave a simple picture of the atom which immediately suggested 
the application of mechanical and electromagnetic ideas. Yet, it was 
clear that, according to classical physical principles, no configuration 
of electrical particles could possess the stability necessary to the 
explanation of the physical and chemical properties of atoms. In 
particular, according to classical electromagnetic theory, every mo- 
tion of the electrons around the atomic nucleus would produce a 
continual radiation of energy implying a rapid contraction of the 
system until the electrons became united with the nucleus into a 
neutral particle of dimensions vanishingly small relative to those 
which must be ascribed to atoms. However, in the hitherto entirely 
incomprehensible empirical laws for the line spectra of the elements 
was found a hint as to the decisive importance of the quantum of 
action for the stability and radiative reactions of the atom. 

The point of departure became here the so-called quantum pos- 
tulate, according to which every change in the energy of an atom 
is the result of a complete transition between two of its stationary 
states. By assuming further that all atomic radiative reactions in- 



volve the emission or absorption of a single light quantum, the energy 
values of the stationary states could be determined from the spectra. 
It was evident that no explanation of the indivisibility of the transi- 
tion processes, or their appearance under given conditions, could be 
given within the framework of deterministic description. However, 
it proved possible 1» obtain a survey of the electron bindings in the 
atom, which reflected many of the properties of substances, with the 
aid of the so-called correspondence principle. On the basis of a 
comparison with the classically expected course of the processes, 
directives were sought for a statistical generalization of the descrip- 
tion compatible with the quantum postulate. Still, it became more 
and more clear that, in order to obtain a consistent account of atomic 
phenomena, it was necessary to renounce even more the use of pic- 
tures and that a radical reformulation of the whole description was 
needed to provide room for all features implied by the quantum of 

The solution which was reached as a result of the ingenious con- 
tributions of many of the most eminent theoretical physicists of 
our time was surprisingly simple. As in the formulation of relativity 
theory, adequate tools were found in highly developed mathematical 
abstractions. The quantities which in classical physics are used to 
describe the state of a system are replaced in quantum-mechanical 
formalism by symbolic operators whose commutability is limited by 
rules containing the quantum. This implies that quantities such as 
positional coordinates and corresponding momentum components of 
particles cannot simultaneously be ascribed definite values. In this 
way, the statistical character of the formalism is displayed as a natural 
generalization of the description of classical physics. In addition, 
this generalization permitted a consequent formulation of the regu- 
larities which limit the individuality of identical particles and which, 
like the quantum itself, cannot be expressed in terms of usual physical 

By means of the methods of quantum mechanics it was possible 
to account for a very large amount of the experimental evidence on 
the physical and chemical properties of substances. Not only was 
the binding of electrons in atoms and molecules clarified in detail, 
but a deep insight was also obtained into the constitution and reac- 
tions of atomic nuclei. In this connection, we may mention that the 
probability laws for spontaneous radioactive transmutations have 
been harmoniously incorporated into the statistical quantum-me- 
chanical description. Also the understanding of the properties of the 
new elementary particles, which have been observed in recent years 



in the study of transmutations of atomic nuclei at high energies, has 
been subject to continual progress resulting from the adaption of the 
formalism to the invariance requirements of relativity theory. Still, 
we are here confronted with new problems whose solution obviously 
demands further abstractions suited to combine the quantum of action 
with the elementary electric charge. 

In spite of the fruitfulness of quantum mechanics within such 
a wide domain of experience, the renunciation of accustomed de- 
mands on physical explanation has caused many physicists and philos- 
ophers to doubt that we are here dealing with an exhaustive descrip- 
tion of atomic phenomena. In particular, the view has been ex- 
pressed that the statistical mode of description must be regarded as a 
temporary expedient which, in principle, ought to be replaceable 
by a deterministic description. The thorough discussion of this 
question has, however, led to that clarification of our position as 
observers in atomic physics which has given us the epistemological 
lesson referred to in the beginning of this lecture. 

As the goal of science is to augment and order our experience, 
every analysis of the conditions of human knowledge must rest on 
considerations of the character and scope of our means of communi- 
cation. Our basis is, of course, the language developed for orienta- 
tion in our surroundings and for the organization of human com- 
munities. However, the increase of experience has repeatedly raised 
questions as to the sufficiency of the concepts and ideas incorporated 
in daily language. Because of the relative simplicity of physical prob- 
lems, they are especially suited to investigate the use of our means 
of communication. Indeed, the development of atomic physics has 
taught us how, without leaving common language, it is possible to 
create a framework sufficiently wide for an exhaustive description of 
new experience. 

In this connection, it is imperative to realize that in every account 
of physical experience one must describe both experimental condi- 
tions and observations by the same means of communication as one 
used in classical physics. In the analysis of single atomic particles, 
this is made possible by irreversible amplification effects— such as a 
spot on a photographic plate left by the impact of an electron, or an 
electric discharge created in a counter device— and the observations 
concern only where and when the particle is registered on the plate 
or its energy on arrival at the counter. Of course, this information 
presupposes knowledge of the position of the photographic plate rela- 


tive to the other parts of the experimental arrangement, such as regu- 
lating diaphragms and shutters defining space-time coordination or 
electrified and magnetized bodies which determine the external force 
fields acting on the particle and permit energy measurements. The 
experimental conditions can be varied in many ways, but the point 
is that in each case we must be able to communicate to others what 
we have done and what we have learned, and that therefore the func- 
tioning of the measuring instruments must be described within the 
framework of classical physical ideas. 

As all measurements thus concern bodies sufficiently heavy to per- 
mit the quantum to be neglected in their description, there is, strictly 
speaking, no new observational problem in atomic physics. The 
amplification of atomic effects, which makes it possible to base the 
account on measurable quantities and which gives the phenomena a 
peculiar closed character, only emphasizes the irreversibility char- 
acteristic of the very concept of observation. While, within the 
frame of classical physics, there is no difference in principle between 
the description of the measuring instruments and the objects under 
investigation, the situation is essentially different when we study 
quantum phenomena, since the quantum of action imposes restric- 
tions on the description of the state of the systems by means of 
space-time coordinates and momentum-energy quantities. Since the 
deterministic description of classical physics rests on the assumption 
of an unrestricted compatibility of space-time coordination and the 
dynamical conservation laws, we are obviously confronted here with 
the problem of whether, as regards atomic objects, such a descrip- 
tion can be fully retained. 

The role of the interaction between objects and measuring instru- 
ments in the description of quantum phenomena was found to be 
especially important for the clarification of this main point. Thus, 
as stressed by Heisenberg, the locating of an object in a limited space- 
time domain involves, according to quantum mechanics, an exchange 
of momentum and energy between instrument and object which is 
the greater the smaller the domain chosen. It was therefore of the 
utmost importance to investigate the extent to which the interaction 
entailed in observation can be taken into account separately in the 
description of phenomena. This question has been the focus of 
much discussion, and there have appeared many proposals which aim 
at the complete control of all interactions. In such considerations, 
however, due regard is not taken to the fact that the very account 
of the functioning of measuring instruments involves that any inter- 



action, implied by the quantum, between these and the atomic ob- 
jects, be inseparably entailed in the phenomena. 

Indeed, every experimental arrangement permitting the registra- 
tion of an atomic particle in a limited space-time domain demands 
fixed measuring rods and synchronized clocks which, from their very 
definition, exclude the control of momentum and energy transmitted 
to them. Conversely, any unambiguous application of the dynamical 
conservation laws in quantum physics requires that the description of 
the phenomena involve a renunciation in principle of detailed space- 
time coordination. This mutual exclusiveness of the experimental 
conditions implies that the whole experimental arrangement must be 
taken into account in a well-defined description of the phenomena. 
The indivisibility of quantum phenomena finds its consequent ex- 
pression in the circumstance that every definable subdivision would 
require a change of the experimental arrangement with the appear- 
ance of new individual phenomena. Thus, the very foundation of a 
deterministic description has disappeared and the statistical character 
of the predictions is evidenced by the fact that in one and the same 
experimental arrangement there will in general appear observations 
corresponding to different individual processes. 

Such considerations not only have clarified the above-mentioned 
dilemma with respect to the propagation of light, but have also com- 
pletely solved the corresponding paradoxes confronting pictorial 
representation of the behaviour of material particles. Here, of 
course, we cannot seek a physical explanation in the customary sense, 
but all we can demand in a new field of experience is the removal of 
any apparent contradiction. However great the contrasts exhibited 
by atomic phenomena under different experimental conditions, such 
phenomena must be termed complementary in the sense that each is 
well defined and that together they exhaust all definable knowledge 
about the objects concerned. The quantum-mechanical formalism, 
the sole aim of which is the comprehension of observations obtained 
under experimental conditions described by simple physical con- 
cepts, gives just such an exhaustive complementary account of a very 
large domain of experience. The renunciation of pictorial represen- 
tation involves only the state of atomic objects, while the foundation 
of the description of the experimental conditions, as well as our free- 
dom to choose them, is fully retained. The whole formalism which 
can be applied only to closed phenomena must in all such respects be 
considered a rational generalization of classical physics. 

In view of the influence of the mechanical conception of nature 
on philosophical thinking, it is understandable that one has sometimes 



seen in the notion of complementarity a reference to the subjective 
observer, incompatible with the objectivity of scientific description. 
Of course, in every field of experience we must retain a sharp dis- 
tinction between the observer and the content of the observations, 
but we must realize that the discovery of the quantum of action has 
thrown new light on the very foundation of the description of nature 
and revealed hitherto unnoticed presuppositions to the rational use of 
the concepts on which the communication of experience rests. In 
quantum physics, as we have seen, an account of the functioning of 
the measuring instruments is indispensable to the definition of phe- 
nomena and we must, so-to-speak, distinguish between subject and 
object in such a way that each single case secures the unambiguous 
application of the elementary physical concepts used in the descrip- 
tion. Far from containing any mysticism foreign to the spirit of 
science, the notion of complementarity points to the logical condi- 
tions for description and comprehension of experience in atomic 

The epistemological lesson of atomic physics has naturally, just as 
have earlier advances in physical science, given rise to renewed con- 
sideration of the use of our means of communication for objective 
description in other fields of knowledge. Not least the emphasis 
placed on the observational problem raises the question of the posi- 
tion of living organisms in the description of nature and of our own 
situation as thinking and acting beings. Even though it was, to some 
extent, possible within the frame of classical physics to compare 
organisms with machines, it was clear that such comparisons did not 
take sufficient account of many of the characteristics of life. The 
inadequacy of the mechanical concept of nature for the description 
of man's situation is particularly evident in the difficulties entailed in 
the primitive distinction between soul and body. 

The problems with which we are confronted here are obviously 
connected with the fact that the description of many aspects of hu- 
man existence demands a terminology which is not immediately 
founded on simple physical pictures. However, recognition of the 
limited applicability of such pictures in the account of atomic phe- 
nomena gives a hint as to how biological and psychological phenomena 
may be comprehended within the frame of objective description. 
As before, it is here important to be aware of the separation be- 
tween the observer and the content of the communications. While, 
in the mechanical conception of nature, the subject-object distinction 

9 2 


was fixed, room is provided for a wider description through the 
recognition that the consequent use of our concepts requires dif- 
ferent placings of such a separation. 

Without attempting any exhaustive definition of organic life, 
we may say that a living organism is characterized by its integrity 
and adaptability, which implies that a description of the internal 
functions of an organism and its reaction to external stimuli often 
requires the word purposeful, which is foreign to physics and chem- 
istry. Although the results of atomic physics have found a multitude 
of applications in biophysics and biochemistry, the closed individual 
quantum phenomena exhibit, of course, no feature suggesting the 
notion of life. As we have seen, the description of atomic phenomena, 
exhaustive within a wide domain of experience, is based on the free 
use of such measuring instruments as are necessary to proper appli- 
cation of the elementary concepts. In a living organism, however, 
such a distinction between the measuring instruments and the ob- 
jects under investigation can hardly be fully carried through, and 
we must be prepared that every experimental arrangement whose 
aim is a description of the functioning of the organism, which is well 
defined in the sense of atomic physics, will be incompatible with the 
display of life. 

In biological research, references to features of wholeness and 
purposeful reactions of organisms are used together with the in- 
creasingly detailed information on structure and regulatory processes 
that has resulted in such great progress not least in medicine. We 
have here to do with a practical approach to a field where the 
means of expression used for the description of its various aspects 
refer to mutually exclusive conditions of observation. In this con- 
nection, it must be realized that the attitudes termed mechanistic 
and finalistic are not contradictory points of view, but rather exhibit 
a complementary relationship which is connected with our position 
as observers of nature. To avoid misunderstanding, however, it is 
essential to note that— in contrast to the account of atomic regulari- 
ties—a description of organic life and an evaluation of its possibilities 
of development cannot aim at completeness, but only at sufficient 
width of the conceptual framework. 

In the account of psychical experiences, we meet conditions of 
observation and corresponding means of expression still further re- 
moved from the terminology of physics. Quite apart from the 
extent to which the use of words like instinct and reason in the 
description of animal behaviour is necessary and justifiable, the 
word consciousness, applied to oneself as well as to others, is indis- 



pensable when describing the human situation. While the terminol- 
ogy adapted to orientation in the environment could take as its start- 
ing point simple physical pictures and ideas of causality, the ac- 
count of our states of mind required a typical complementary mode 
of description. Indeed, the use of words like thought and feeling 
does not refer to. a firmly connected causal chain, but to experiences 
which exclude each other because of different distinctions between 
the conscious content and the background which we loosely term 

The relation between the experience of a feeling of volition and 
conscious pondering on motives for action is especially instructive. 
The indispensability of such apparently contrasting means of ex- 
pression to the description of the richness of conscious life strik- 
ingly reminds us of the way in which elementary physical concepts 
are used in atomic physics. In such a comparison, however, we must 
recognize that psychical experience cannot be subjected to physical 
measurements and that the very concept of volition does not refer 
to a generalization of a deterministic description, but from the out- 
set points to characteristics of human life. Without entering into the 
old philosophical discussion of freedom of the will, I shall only men- 
tion that in an objective description of our situation use of the word 
volition corresponds closely to that of words like hope and responsi- 
bility, which are equally indispensable to human communication. 

We have here reached problems which touch human fellowship 
and where the variety of means of expression originates from the 
impossibility of characterizing by any fixed distinction the role of 
the individual in the society. The fact that human cultures, devel- 
oped under different conditions of living, exhibit such contrasts with 
respect to established traditions and social patterns allows one, in a 
certain sense, to call such cultures complementary. However, we 
are here in no "way dealing with definite mutually exclusive fea- 
tures, such as those we meet in the objective description of general 
problems of physics and psychology, but with differences in atti- 
tude which can be appreciated or ameliorated by extended inter- 
course between peoples. In our time, when increasing knowledge 
and ability more than ever link the fate of all peoples, international 
collaboration in science has far-reaching tasks which may be furthered 
not least by an awareness of the general conditions for human 

Physical Science 


the Problem of Life 

It has been a pleasure to accept the invitation of the Medical 
Society of Copenhagen to give one of the Steno lectures by which 
the Society commemorates the famous Danish scientist whose achieve- 
ments are admired in ever greater measure, not only in this country 
but in the whole scientific world. As my theme I have chosen 
a problem which has occupied the human mind through the ages 
and with which Niels Stensen himself was deeply concerned, namely 
how far physical experience can help us to explain organic life in its 
rich and varied display. As I shall try to show, the development of 
physics in recent decades and in particular the lesson regarding our 
position as observers of that nature of which we are part, received 
through the exploration of the world of atoms so long closed to us, 
have created a new background for our attitude to this question. 

Even in the philosophical schools of ancient Greece, we find 
divergent opinions regarding the conceptual means suited to account 
for the striking differences between living organisms and other ma- 
terial bodies. As is well known, the atomists considered a limited 
divisibility of all matter necessary to explain not only simple physical 
phenomena but also the functioning of organisms and the related 




psychical experiences. Aristotle, on the other hand, refuted atomic 
ideas and, in view of the wholeness exhibited by every living or- 
ganism, maintained the necessity of introducing into the description 
of nature such concepts as perfection and purposefulness. 

For almost 2000 years the situation remained essentially unchanged, 
and not until the Renaissance did there occur the great discoveries 
in physics as well as in biology, which were to give new incentives. 
Progress in physics consisted above all in the liberation from the 
Aristotelian idea of driving forces as the cause of all motion. Gal- 
ileo's recognition that uniform motion is a manifestation of inertia 
and his emphasis on force as a cause of change of motion were to 
become the foundation of the development of mechanics, which 
Newton to the admiration of succeeding generations endowed with a 
firm and completed form. In this so-called classical mechanics all 
reference to purpose is eliminated, since the course of events is 
described as automatic consequences of given initial conditions. 

The progress of mechanics could not avoid making the strongest 
impression on all contemporary science. In particular, the anatomical 
studies of Vesalius and Harvey's discovery of the circulation of the 
blood suggested the comparison between living organisms and ma- 
chines working according to the laws of mechanics. On the philo- 
sophical side, it was especially Descartes who stressed the similarity 
between animals and automata, but ascribed to human beings a soul 
interacting with the body in a certain gland in the brain. However, 
the insufficiency of contemporary knowledge of such problems was 
emphasized by Steno in his famous Paris lecture on the anatomy of 
the brain, which bears witness of the great observational power and 
open-mindedness characteristic of all Steno's scientific work. 

Subsequent developments in biology, especially after the invention 
of the microscope, revealed an unsuspected fineness in organic struc- 
ture and regulatory processes. At the same time that mechanis- 
tic ideas thus found ever wider applications, so-called vitalistic or 
finalistic points of view, inspired by the wonderful power of regen- 
eration and adaptation in organisms, were repeatedly expressed. 
Rather than returning to primitive ideas of a life force acting in the 
organisms, such views emphasized the insufficiency of physical ap- 
proach in accounting for the characteristics of life. As a sober pres- 
entation of the situation as it stood in the beginning of this century, 
I should like to refer to the following statement by my father, the 
physiologist Christian Bohr, in the introduction to his paper "On 
Pathological Lung Expansion" which appeared in the anniversary 
publication of the Copenhagen University in 1910. 

96 atomic physics and human knowledge 

As far as physiology can be characterized as a special branch of natural 
sciences, its specific task is to investigate the phenomena peculiar to the 
organism as a given empirical object in order to obtain an understanding 
of the various parts in the self-regulation and how they are balanced 
against each other and brought into harmony with variations in external 
influences and inner processes. It is thus in the very nature of this task to 
refer the word purpose to the maintenance of the organism and consider 
as purposive the regulation mechanisms which serve this maintenance. Just 
in this sense we shall in the following use the notion "purposiveness" about 
organic functions. In order that the application of this concept in each 
single case should not be empty or even misleading it must, however, be 
demanded that it be always preceded by an investigation of the organic 
phenomenon under consideration, sufficiently thorough to illuminate step 
by step the special way in which it contributes to the maintenance of the 
organism. Although this demand, which requires no more than the scien- 
tific demonstration that the notion of purposiveness in the given case is 
used in accordance with its definition, might appear self-evident, it may 
nevertheless not be unnecessary to stress it. Indeed, physiological investi- 
gations have brought to light regulations of utmost fineness in a multitude 
so great that it is a temptation to designate every observed manifestation 
of life as purposive without attempting an experimental investigation of its 
detailed functioning. By means of analogies which so easily present them- 
selves among the variety of organic functions, it is merely the next step 
to interpret this functioning from a subjective judgement about the special 
character of purposiveness in the given case. It is evident, however, how 
often, with our so narrowly limited knowledge about the organism, such 
a personal judgement may be erroneous, as is illustrated by many examples. 
In such cases, it is the lacking experimental illumination of the details of 
the process which is the cause of the erroneous results of the procedure. 
The a priori assumption of the purposiveness of the organic process is, 
however, in itself quite natural as a heuristic principle and can, due to the 
extreme complication and difficult comprehension of the conditions in the 
organism, prove not only useful, but even indispensable for the formula- 
tion of the special problem for the investigation and the search of ways 
for its solution. But one thing is what may be conveniently used by the 
preliminary investigation, another what justifiably can be considered an 
actually achieved result. As regards the problem of the purposiveness 
of a given function for the maintenance of the whole organism, such a 
result can, as stressed above, be secured only by a demonstration in detail 
of the ways in which the purpose is reached. 

I have quoted these remarks which express the attitude in the 
circle in which I grew up and to whose discussions I listened in my 
youth, because they offer a suitable starting point for the investigation 
of the place of living organisms in the description of nature. As I 
shall try to show, modern development of atomic physics, at the same 
time as it has augmented our knowledge about atoms and their con- 
stitution of more elementary parts, has revealed the limitation in 



principle of the so-called mechanical conception of nature and 
thereby created a new background for the problem, most pertinent 
to our subject, as to what we can understand by and demand of a 
scientific explanation. 

In order to present the situation in physics as clearly as possible, I 
shall start by reminding you of the extreme attitude which, under the 
impact of the great success of classical mechanics, was expressed in 
Laplace's well-known conception of a world machine. All interac- 
tions between the constituents of this machine were governed by the 
laws of mechanics, and therefore an intelligence knowing the rela- 
tive positions and velocities of these parts at a given moment could 
predict all the subsequent events in the world, including the be- 
haviour of animals and man. In this whole conception which has, 
as is well known, played an important role in philosophical discussion, 
due attention is not paid to the presuppositions for the applicability 
of the concepts indispensable for communication of experience. 

In this respect the later development of physics has given us an 
urgent lesson. Already the far-reaching interpretation of heat phe- 
nomena as incessant motion of molecules in gases, liquids, and solids 
has called attention to the importance of the conditions of observa- 
tion in the account of experience. Of course, there could be no ques- 
tion of a detailed description of the motions of the innumerable parti- 
cles among each other, but only of deducing statistical regularities of 
heat motion by means of general mechanical principles. The peculiar 
contrast between the reversibility of simple mechanical processes and 
the irreversibility typical of many thermodynamical phenomena was 
thus clarified by the fact that applications of such concepts as tem- 
perature and entropy refer to experimental conditions incompatible 
with complete control of the motions of single molecules. 

In the maintenance and growth of living organisms one has often 
seen a contradiction to that tendency, implied by the thermodynami- 
cal laws, towards temperature and energy equilibrium in an isolated 
physical system. However, we must remember that organisms are 
continually supplied with free energy by nutrition and respiration, 
and the most thorough physiological investigation has never revealed 
any departure from thermodynamical principles. Yet, recognition 
of such similarities between living organisms and ordinary power 
engines is of course in no way sufficient to answer the question about 
the position of organisms in the description of nature, a question 
obviously demanding deeper analysis of the observational problem. 


This very problem has indeed been brought to the foreground in 
an unexpected way by the discovery of the universal quantum of 
action which expresses a feature of wholeness in atomic processes 
that prevents the distinction between observation of phenomena 
and independent behaviour of the objects, characteristic of the me- 
chanical conception of nature. In physical systems on the ordinary 
scale, representation of events as a chain of states described by 
measurable quantities rests on the circumstance that all actions are 
here large enough to permit neglect of the interaction between the 
objects and the bodies used as measuring tools. Under conditions 
where the quantum of action plays a decisive part and where such 
an interaction is therefore an integral part of the phenomena, there 
cannot to the same extent be ascribed a mechanically well-defined 

The breakdown of ordinary physical pictures which here confronts 
us is strikingly expressed in the difficulties in talking about properties 
of atomic objects independent of the conditions of observation. In- 
deed, an electron may be called a charged material particle, since 
measurements of its inertial mass always give the same result, and any 
transmission of electricity between atomic systems always amounts to 
a number of so-called unit charges. Yet, the interference effects ap- 
pearing when electrons pass through crystals are incompatible with 
the mechanical idea of particle motion. We meet analogous features 
in the well-known dilemma about the nature of light, since optical 
phenomena require the notion of wave propagation, while the laws 
of transmission of momentum and energy in atomic photo-effects 
refer to the mechanical conception of particles. 

This situation, novel in physical science, has demanded a renewed 
analysis of the presuppositions for the application of concepts used 
for orientation in our surroundings. Of course, in atomic physics 
we retain the freedom by experimenting to put questions to nature, 
but we must recognize that the experimental conditions which can 
be varied in numerous ways are defined only by bodies so heavy that 
in the description of their functions we can disregard the quantum. 
Information concerning atomic objects consists solely in the marks 
they make on these measuring instruments, as, for instance, a spot 
produced by the impact of an electron on a photographic plate 
placed in the experimental arrangement. The circumstance that such 
marks are due to irreversible amplification effects endows the phe- 
nomena with a peculiarly closed character pointing directly to the 
irreversibility in principle of the very notion of observation. 



The special situation in quantum physics is above all, however, 
that the information gained about atomic objects cannot be compre- 
hended along the lines of approach typical of the mechanical con- 
ception of nature. Already the fact that under one and the same 
experimental arrangement there may in general appear observations 
pertaining to different individual quantum processes entails a limita- 
tion in principle of the deterministic mode of description. The de- 
mand of unrestricted divisibility on which classical physical descrip- 
tion rests is also clearly incompatible with that feature of wholeness 
in typical quantum phenomena which involves that any definable 
subdivision requires a change of the experimental arrangement giving 
rise to new individual effects. 

In order to characterize the relation between phenomena observed 
under different experimental conditions, one has introduced the term 
complementarity to emphasize that such phenomena together ex- 
haust all definable information about the atomic objects. Far from 
containing any arbitrary renunciation of customary physical ex- 
planation, the notion of complementarity refers directly to our posi- 
tion as observers in a domain of experience where unambiguous ap- 
plication of the concepts used in the description of phenomena de- 
pends essentially on the conditions of observation. By a mathemati- 
cal generalization of the conceptual framework of classical physics it 
has been possible to develop a formalism which leaves room for the 
logical incorporation of the quantum of action. This so-called quan- 
tum mechanics aims directly at the formulation of statistical regulari- 
ties pertaining to evidence gained under well-defined observational 
conditions. The completeness in principle of this description is due 
to the retention of classical mechanical ideas to an extent including 
any definable variation of the experimental conditions. 

The complementary character of the quantum-mechanical descrip- 
tion is clearly expressed in the account of the composition and reac- 
tions of atomic systems. Thus, the regularities regarding the energy 
states of atoms and molecules, responsible for the characteristic spec- 
tra of the elements and the valences of chemical combinations, appear 
only under circumstances where a control of the positions of the 
electrons within the atom and the molecule is excluded. In this con- 
nection, it is interesting to note that fruitful application of the struc- 
ural formulae in chemistry rests solely on the fact that the atomic 
nuclei are so much heavier than the electrons. However, with respect 
to the stability and transmutations of the atomic nuclei themselves, 
quantum-mechanical features are again decisive. Only in a com- 
plementary description transcending the scope of the mechanical 



conception of nature is it possible to find room for the fundamental 
regularities responsible for the properties of the substances of which 
our tools and our bodies are composed. 

Progress in the field of atomic physics has, as is well known, found 
wide application in the biological sciences. In particular, I may 
mention the understanding we have gained of the peculiar stability 
of chemical structures in the cells responsible for the hereditary prop- 
erties of the species, and of the statistical laws for the occurrence 
of mutations produced by exposing organisms to special agencies. 
Furthermore, amplification effects similar to those permitting observa- 
tion of individual atomic particles play a decisive role in many func- 
tions of the organism. In this way is stressed the irreversible char- 
acter of typical biological phenomena, and the time direction inherent 
in the description of the functioning of organisms is strikingly marked 
by their utilization of past experience for reactions to future stimuli. 

In this promising development we have to do with a very impor- 
tant and, according to its character, hardly limited extension of the 
application of purely physical and chemical ideas to biological prob- 
lems, and since quantum mechanics appears as a rational generalization 
of classical physics, the whole approach may be termed mechanistic. 
The question, however, is in what sense such progress has removed the 
foundation for the application of so-called finalistic arguments in 
biology. Here we must realize that the description and comprehen- 
sion of the closed quantum phenomena exhibit no feature indicating 
that an organization of atoms is able to adapt itself to the surroundings 
in the way we witness in the maintenance and evolution of living or- 
ganisms. Furthermore, it must be stressed that an account, exhaustive 
in the sense of quantum physics, of all the continually exchanged 
atoms in the organism not only is infeasible but would obviously re- 
quire observational conditions incompatible with the display of life. 

However, the lesson with respect to the role which the tools of 
observation play in defining the elementary physical concepts gives 
a clue to logical application of notions like purposiveness foreign to 
physics, but lending themselves so readily to the description of or- 
ganic phenomena. Indeed, on this background it is evident that the 
attitudes termed mechanistic and finalistic do not present contradic- 
tory views on biological problems, but rather stress the mutually ex- 
clusive character of observational conditions equally indispensable in 
our search for an ever richer description of life. Here, there is of 
course no question of an explanation akin to the classical physical ac- 
count of the functioning of simple mechanical constructions or of 



complicated electron calculation machines, but we are concerned 
with wider pursuit of that analysis of the presuppositions and scope of 
our conceptual means of communication which has become so char- 
acteristic of the newer development of physics. 

Apart from all differences with respect to observational conditions, 
communication of biological experiences contains no more reference 
to the subjective observer than does the description of physical evi- 
dence. Thus, so far it has not been necessary to enter more closely 
into the conditions of observation characteristic for the account of 
psychological phenomena, for which we cannot rely on the con- 
ceptual frame developed for our orientation in inanimate nature. 
However, the fact that conscious experience can be remembered and 
therefore must be supposed to be connected with permanent changes 
in the constitution of the organism points to a comparison between 
psychical experiences and physical observations. With respect to 
relationships between conscious experiences we also encounter fea- 
tures reminiscent of the conditions for the comprehension of atomic 
phenomena. The rich vocabulary used in the communications of the 
states of our mind refers indeed to a typical complementary mode 
of description corresponding to a continual change of the content on 
which attention is focused. 

Compared to the extension of the mechanical mode of description 
demanded by the account of the individuality of atomic phenomena, 
the integrity of the organism and the unity of the personality confront 
us of course with a further generalization of the frame for the rational 
use of our means of communication. In this respect, it must be em- 
phasized that the distinction between subject and object, necessary 
for unambiguous description, is retained in the way that in every 
communication containing a reference to ourselves we, so-to-speak, 
introduce a new subject which does not appear as part of the content 
of the communication. It need hardly be stressed that it is just this 
freedom of choosing the subject-object distinction which provides 
room for the multifariousness of conscious phenomena and the rich- 
ness of human life. 

The attitude to general problems of knowledge to which the de- 
velopment of physics in this century has led us differs essentially from 
the approach to such problems at Steno's time. This does not mean, 
however, that we have left the road to the enrichment of knowledge 
followed by him with such great results, but we have realized that the 
striving for beauty and harmony which marked Steno's work demands 
a steady revision of the presuppositions and scope of our means of